Close #81

Close #80

Including mention of Library Carpentry in include files

Merge last changes on https://github.com/swcarpentry/styles/

Knit episodes only when any one RMarkdown source changes

Adding Google Analytics

add more information about pkgs needed to build the lesson

Unifying links in a single file

Adding comments on purposes of include files

Using page.previous and page.next for links instead of loop

lesson.scss: 2em left padding for lists

Auto-generating IDs for glossary items

Fixing layout of descriptions of standard pages

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.

Change target of setup link from .md to site page

The link worked fine, but directing people to the setup.md file instead of the rendered page means that other links don't work - such as item 12 in the setup page: https://github.com/swcarpentry/lesson-example/blob/gh-pages/setup.md

Update a few files to make this directly readable on github

Example of the problem: see "Ten Things You Need To Know" from http://swcarpentry.github.io/lesson-example/
For numbered lists that have more that 9 elements 1 em padding on the left is not sufficient. The result is that "1" in numbers "10" and above is printed on top of the border of encapsulating HTML block. 2 em padding solves this problem for lists with less that 100 elements. It should be safe to safe to assume that numbered lists in Carpentries' lessons should not have more than 99 items.