Make docs logo text a theme configuration option

This replaces the logo in the top left corner of the sphinx theme with just the icon (rather than the icon plus words). The words are now part of the html, and google web fonts is used to supply the "Source Sans Pro" font we use for the logo.

Additionally, the text is a theme configuration option. In `docs/conf.py`, you can add this:

    html_theme_options = {
        'logotext1': 'hello',
        'logotext2': 'world',
        'logotext3': ''
        }

and you'll get this:

![Screenshot from 2013-02-25 23:05:29](https://f.cloud.github.com/assets/970135/195112/adcbd444-7fd7-11e2-9fa5-128f8a67ac7b.png)

The only downside I see is that if you are viewing the docs offline, the logo might look crappier (the font will fallback to 'sans-serif').