Added announcement template

Design: https://projects.invisionapp.com/d/main#/console/1392015/34940936/preview

Homepage: 
![screen shot 2014-10-17 at 12 59 55 pm](https://cloud.githubusercontent.com/assets/910510/4682681/9cd744dc-561f-11e4-807d-8502534a1f15.png)


Course page: 
![screen shot 2014-10-17 at 1 03 39 pm](https://cloud.githubusercontent.com/assets/910510/4682682/a202e970-561f-11e4-8781-18fef19320ef.png)


@marcotuts please advice on the design for the course pages as well as the dismiss icon/link.

@dsjen @rocha @shnayder 
Shouldn't we have the CSRF token added at a higher level, for example `base.html`?