Course

Implemented back-end part of courses and checkpoints with markdown support.
To display rendered to HTML markdown in views use description_html method of Course or Checkpoint models.

To embed youtube videos into markdown use the following syntax:
```
![WIDTHxHEIGHT](youtube:YOUTUBEID)
```
for example
```
![450x230](youtube:qCW-SVPCw4Y)
```
Width and height are optional, I've set defaults to 560x315, you can change them in lib/markdown.rb in MediaRenderer#youtube_link method.

Markdown already supports embedding images like this:
```
![alt text](http://example.com/image.jpg)
```
course can be seen by public, only checkpoints can be seen if purchased the course