Make the version number PEP 440 compliant

Building an egg from a development version of traits was causing bad version numbers (which are then corrected by newer egg infrastructure). Anyhow, it's a mess. There need to be a number after the `.dev` part of the version. This adds that using code from enstaller.
There's an inconsistency in types: in this branch `git_revision` has type `str`, while in the previous branch it has type `unicode`.