centralise my_courses

As addressed by @raymondtangsc, after changing my_courses from thumbnail to table, the UI looks very weird on large screen. This is because the whole view is left aligned and the table will not extend to the right as thumbnails, which will left a blank area on the right.

Fixed this by centralise the whole my_courses content.

Before (on large screen):
![screen shot 2014-12-20 at 5 16 35 pm](https://cloud.githubusercontent.com/assets/4983239/5514366/06d55166-886c-11e4-8728-652086e81b03.png)

Now:
![screen shot 2014-12-20 at 5 16 09 pm](https://cloud.githubusercontent.com/assets/4983239/5514367/16820ea6-886c-11e4-91e5-6eb5cbe9b1fa.png)