New documentation with DFM's sphinx theme

Adapted @dfm's sweet sphinx theme from encee to use here. Using Adobe
Source Sans and Code via Google Fonts. Using a retina-ready github fork
ribbon.

The documentaion content is a bit more elaborate. Made a separate page
for installation instructions. Two API references pages for the
StellarPopulation class and fsps.filters module, respectively. These
pages start with an example, then do sphinx-autodoc API.
Also included a table of FSPS filters generated with the
fsps_filter_table.py script.
Merge pull request #17 from dfm/docs

New documentation with DFM's sphinx theme