Added Basic Branding Support

The logo is now easily re-branded. Updated README to include info for additional customization. The Open edX theme is now the default. Below are some examples of customizations for Stanford.

![screen shot 2014-11-14 at 2 01 28 pm](https://cloud.githubusercontent.com/assets/910510/5052341/959a94c2-6c0d-11e4-82e4-33e212c0e85b.png)
![screen shot 2014-11-14 at 2 03 46 pm](https://cloud.githubusercontent.com/assets/910510/5052338/9597c7a6-6c0d-11e4-879b-60c5c57e52d6.png)
![screen shot 2014-11-14 at 2 09 35 pm](https://cloud.githubusercontent.com/assets/910510/5052342/959c695a-6c0d-11e4-82ea-14d0f01709a7.png)
![screen shot 2014-11-14 at 2 10 24 pm](https://cloud.githubusercontent.com/assets/910510/5052340/95996e8a-6c0d-11e4-8bc2-040c143d03df.png)
![screen shot 2014-11-14 at 2 43 01 pm](https://cloud.githubusercontent.com/assets/910510/5052343/959d9cee-6c0d-11e4-99e4-f57c2ab9fd51.png)
![screen shot 2014-11-14 at 2 49 44 pm](https://cloud.githubusercontent.com/assets/910510/5052339/959932ee-6c0d-11e4-91f6-b85d7b70335a.png)

@rocha @dsjen @shnayder @marcotuts @jbau 
These constants, actually, are specific to the edX logo. They belong here.