consolidate badge classes to swc.css

Thanks to ocefpaf for request that.

Add variable to represent Pandoc

Pandoc had some changes in the last versions.
Having a variable for Pandoc will help
maintainers to handle different versions of Pandoc.

Figures should be centered
and not overflow the screen.

Add article tag as wrap for lesson content

CONTRIBUTING.md: Remove the maintainer references

Because we're going to be storing maintainer information locally in
the README.md [1].

[1]: https://github.com/swcarpentry/lesson-template/issues/195#issuecomment-89643048

Remove logger name (root) from console messages

Ignore conduct.md and contributing.md. No, really.

(even though the contents are not validated)

tools: Add trailing newlines to files that were missing them

This avoids things like:

  ~$ cat tools/validation_helpers.py
  ...
          return self.is_block(ast_node) and has_heading~$

with your next prompt way over to the right ;).

CONTRIBUTING.md: Use a generic reference name for the issues link

Downstream lesson maintainers are supposed to adapt this to point at
their repository's issues [1], so use a reference name that works for
both the lesson-template and downstream repositories.  That way they
only have to update the referenced URL and can leave the reference
name alone.

[1]: https://github.com/swcarpentry/lesson-template/issues/195#issuecomment-78310923

the class removed are unused as we take advantage of classes provided by
Bootstrap instead.

Bring in style file changes

Add cards class

Add documentation about card class

Render specific blockquotes as Bootstrap panels

Better style for challenges

Fix background for lessons

There's a note in
https://github.com/swcarpentry/lesson-template/pull/182
explaining why we need to fix style for panel heading.

This makes some things look nicer (e.g. adds gradients, etc).

From http://getbootstrap.com/components/#panels-alternatives
the only panel that the font color is white is
`panel-primary` and for that reason it is the only one that we can't use.

This replace the use of `panel-primary` with `panel-warning`
since prerequisites and learning objectives **never**
appear in the same file.