Revamp setup.py, bump version to 2.0b1

core is no longer an alpha level project, beta is more accurate.
Added various classifiers for eventual pypi classification

Users can now access the version with pywikibot.version, which
is standard Python convention. For version.py, this is added
as the "release version".

Change-Id: I1a58a4eceb77215d4aadb16e0708da03013e7668