Signed-off-by: Maxim Belkin <maxim.belkin@gmail.com>

The link to setup is broken. This pull request suggest to link to the existing setup page (`{{ page.root}}/setup.html`), but other options are 
 to change the permalink on Setup to `{{ site.url }}/setup/`

Close #80

1.  Create file `_includes/links.md` with definitions of all links.
2.  Replace links at the bottom of Jekyll'd `.md` files with inclusion of that file.
3.  Modify `bin/lesson_check.py` to load and check definitions in this file.

This eliminates most redundancy in link definitions, though
`CONTRIBUTING.md` and `README.md` still contain their own links, since
they are not Jekyll'd.  Note that we *cannot* put the links into
`_layouts/*.html` (or even `_layouts/something.md`) because of the
order in which Jekyll expands things - we have to explicitly include
it in each page.

All classes **must** be explicit because this will make easy
to translate the lessons.

Template changes to work with validator according to current link validation rules. Also update HTML output.

Add validation methods and refactor ast helpers to separate class

Add tiny logging message; prep for first push to github fork

Start adding tests

Fix errors revealed by unit tests

Update validator to check heading order; tests passing

Validation of links and changes to heading validation

Tiny comment cleanup; push to share version with swc

Add some descriptive comments and create minimal validator example (for instructors.md) to demonstrate subclassing

Split helpers to separate file and update for newest index.md template

Grooming

Bring up to spec with newest versions of lesson templates. Add tests to ensure all templates pass validation.

Saner default assumption about markdown file locations

Add skeletons for remaining validators. Incorporate ranier's code for batch validation.

Code changes for single-file python 2 and 3 compatibility.

Refactor out template validation logic in prep for cmd line rework

Proof of concept refactor to command line sig for @rgaia-cs (issue #34)

With no path provided, default to curdir, not a hard-coded default

Implement license and discussion validators. Some test cleanup.

Validate file links against page titles. Add line length check.

Fix python3 hash error

Rename validator to check

Conflicts:
	tools/validate_markdown_template.py

Enforce section heading levels + pep8 cleanup

Improved link validation for non-html files

Validate images as well as links

Verify that all headings are exactly level 2

Link text validation based on document header "subtitle" instead of "title".

Adjustments based on run against the SQL repo.

Fix links to topics

By default only show erros and warnings to user

- Add another argument for CLI to enable debug log level
- Change some log message to debug instead of info
- Add file information for errors and warnings

Fix import at check

From [PEP8](https://www.python.org/dev/peps/pep-0008/#imports)

> Imports should usually be on separate lines, e.g.:
>
> Yes: import os
>      import sys
>
> No:  import sys, os

Heading bugfixes, update failing tests, and cleanup. fix swcarpentry/lesson-template#39, swcarpentry/lesson-template#42 .

Add validation of glossary

Slight clarification to log messages.

Skip validate some links in index

Fix super() call for python2; avoid mutable default argument.

Lesson template validator.

* Add `.nojekyll` to stop GitHub trying to compile.
* Get rid of Jekyll `_config.yml`.
* Create placeholder `css` and `img` directories.
* Create `pages` directory with source for web pages.
* Move all Markdown files into `pages`.
* Move `tools` into `pages` (because that's where people will mostly work).
* Move `_includes` and `_layouts` into `pages` (because that's where we'll compile).