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.
Sorry; not thinking with my Python 3 hat on here;  I guess we need code that's going to work without change under both Python 2 and Python 3 for the `setup.py`.  (Everything else gets 2-to-3d.)

It's very tempting to drop Python 3.2 support just so we can reasonably use the `u` prefixes for code which needs to be both Python 2 and Python 3 compatible.  But that's outside the scope of this PR.