Updating and moving parameters around

Here is the simplified version of PR #24.  This moves many of the StellarPopulation parameters (mostly switches like add_dust_emission and redshift_colors) from initialization to the ParameterSet, since these parameters are not actually used in driver.setup and can be changed safely after instantiation. This should be backwards compatible with code that used these initialization parameters at instantiation. This will also keep python-FSPS nearly as flexible as FSPS. This also fixes a bug whereby changes in dust_type were ignored and the default (dust_type=0) was always used.

It also adds a number of parameters and switches related to nebular emission, igm attenuation, and vacuum to air conversions, and documentation for these parameters.

The FSPS accepted revisions compatible with all these new parameters is now only r171.