Refactor out a filters module

The StellarPopulation class is long enough for one module.
PEP8 fixes

Keep the syntastic machine happy.
Refactor filter table

Rather than maintain a huge list, in python code, of filter names and
comments, lets just use the filter comments shipped with FSPS itself.
The filter names are unique to python-fsps, and stored separately in a
filter_keys.txt. The two data sets are merged and turned into a
dictionary of Filter instances with the load_filter_dict() function.

Hopefully this will make the filter table easier to maintain
Be defensive about FSPS MSUN and LAMBDA_EFF tables

It seems that FSPS's MSUN and effective wavelength tables may be missing
some filters. We attempt to carry on by returning NaN when a filter is
not included in MSUN or effective wavelength tables for the msun_vega,
msun_ab and lambda_eff properties of Filter.
Script to generate reST filter table

This table can eventually be included in the documentation. Otherwise,
it is a useful way of verifying consistency of filter names and
descriptions in the filter database.
setup.py installs scripts
get_filters() and list_filters()

These functions are part of the public API, allowing a user access to
Filter() instances without having to use the FILTERS dictionary.
Add Filter.transmission property

This gives the user access to FSPS's un-normalized filter transmission
tables ($SPS_HOME/data/all_filters.dat).

In demos/plot_sdss_2mass_transmission.py I generate a sweet plot of the
SDSS+2MASS filter transmissions.
Merge pull request #14 from jonathansick/filter_refactor

Filter refactor
get_mags() returns mags in same order as bands arg

Before, if the user specified the bands argument to
StellarPopulation.get_mags() then the subset of magnitudes would be
returned in the order of FSPS filter indices. The user would probably
expect the mags to match the order of the bands list since the FSPS band
order is rather arbitrary. This commit satisfies that.
Merge pull request #16 from dfm/fix_get_mags

get_mags() returns mags in same order as bands arg
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
links and formatting
simple formatting changes
Fix links in sidebar

Oops.
documentation readme
Merge branch 'master' of https://github.com/dfm/python-fsps
formatting typo
trying to resolve conflicts