Renamed package name to synphot, so that it will be astropy.synphot

Renamed package name to `synphot`, so that it will be `astropy.synphot` when merged into `astropy`. Reorganized codes to how `astropy` is organized, so that transition from affiliated package to sub-package is easier. Renamed `pysynexcept` module to `synexceptions` for clarity. Disabled 'test_simple.py' because any test will not work until codes are completely refactored (ongoing work in a different branch).

p/s: At least version test still passes! Version test = 1, Gandalf = 0