more setup.py fixes

[wholmgren]

fixing #181 led me to a few more problems with our setup.py file:

1. AUTHORS should be something like 'PVLIB Python developers' instead of naming individuals.
2. The current LONG DESCRIPTION results in a poorly formatted PyPI page. Probably fix this after update readme to include only the most important information #182.
3. The version check can be deleted. It's not helping anyone.
4. DESCRIPTION has a typo 'set functions' --> 'set of functions'
5. Move 'install_requires' from setuptools_kwargs to explicit kwarg
6. Add 'tests_require' = ['nose'] kwarg
7. LICENSE doesn't need 'license'