New SolarSysObservable, and make comets and asteroids subclass it.

SolarSysObservable is a SolarSysObject that emits light via the fluxd
method.
Merge SolarSysObject and SolarSysObservable, add lightcurve method.
whitespace fix.
Merge pull request #4 from mkelley/lightcurve

Add the ability for SolarSysObject to estimate fluxes.