New look for homepage
Initial WordPress commit
Added course and checkpoint model
Added markdown extension for youtube

the syntax to embed a youtube video is

![WIDTHxHEIGHT](youtube:YOUTUBEID)

For example: ![450x230](youtube:qCW-SVPCw4Y)
will generate following HTML:
<p><iframe width="450" height="230"
src="//www.youtube.com/embed/qCW-SVPCw4Y" frameborder="0" allowfullscreen></iframe></p>

width and height are optional, you can leave the brackets empty, then
the default size would be 560x315

To render markdown just call Markdown.render(your_markdown)
Added courses controller
Added order column to checkpoint
Added course-checkpoint association, validations, checkpoint order,
rendering markdown to html (description_html method)
Added course_id to purchases
Added price to courses
Added course controller, mailer, and some abilities

Haven't tested it yet.
DRYed up courses controller a bit, added missing create action
Added purchases association to course model
Added checkpoints controller
Added course abilities and course_creator role
Fixed bug with moving all checkpoints on each save
Added friendly ids to courses and checkpoints
Removed useless comment
Added simple views
Courses viewable by everyone
Added missing ability to index courses by all users