Added analytic spectra classes that use astropy.modeling and related tests. Minor codes and docs updates.
Fixed Python 3.3 contiguous array error. Modified BlackBody1DModel to work properly with latest astropy.modeling changes.
Replaced OverlapError with PartialOverlap for exceptions involving partially overlapping spectra.
Replaced local wave_conversion equivalencies with astropy spectral().
Updated flux conversion to use astropy spectral_density() equivalencies when possible.
Moved flux conversion from spectrum.py back to units.py and hopefully has no circular import error on Python 3.
Added analytic to convenience UI import.
Sync with astropy doc organization that replaced _generated with api directory.
Included long C-extension docstring in documentation.
Fix Travis failure for Python 3 because of xrange
Merge pull request #48 from pllim/analytic-models

Analytic models etc