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