Added info message on load
Added renorm test for zero-flux spectrum.
Corrected countrate results to have unit of count/s/cm^2 like IRAF SYNPHOT.
Avoid OSError when config is given a fake dir, only to raise IOError when explicitly requested.
Added EMISSITIVITY to recognized units. Ported overflow/underflow handling for Planck function from old code.
Force blackbody calculation to be done in float64
Class inheriting via super now.
Added thermal spectrum and related tests.
wave_range result does not have to be 2 separate Quantities.
Merge pull request #55 from pllim/st-refac

Improvement to `pysynphot` based on `stsynphot` refactoring work.

Changes visible to end-user:
* Blackbody calculations now handle underflow and overflow.
* Added thermal spectrum class.
* Count rate is now in the correct unit.
* EMISSIVITY (thermal spectrum unit) is now recognized.