Added announcement template

Design: https://projects.invisionapp.com/d/main#/console/1392015/34940936/preview

Homepage: 
![screen shot 2014-10-17 at 11 11 48 am](https://cloud.githubusercontent.com/assets/910510/4681106/58947056-5610-11e4-983c-e0f9890ce770.png)

Course page: 
![screen shot 2014-10-17 at 11 11 43 am](https://cloud.githubusercontent.com/assets/910510/4681110/5d8265e6-5610-11e4-899c-d9a215b693d9.png)

@marcotuts please advice on the design for the course pages.

@dsjen @rocha @shnayder