Refactor codes

Revamped STScI portion of legacy `pysynphot` to use Astropy version. Added Python 3 compatibility. Added unit tests. Renamed some classes and functions to be PEP8 compliant and to make them sound more intuitive or familiar to IRAF SYNPHOT user (within reason). Added initial Sphinx documentation layout. Merged some modules (e.g., `obsbandpass` and `spectrum`). Things like `setref` are now handled inside `config` module (which I think sounds more intuitive than `refs`). Cache is now handled in the relevant modules instead of `locations`, which is retired.