[Python-Dev] Changes to the installer builder

["Martin v. Löwis"]

I have added some documentation to msi.py, please let me know if
something is still missing.

I have also rearranged the versioning. msi.py now supports two
"product lines": snapshots, and releases. Snapshots can be generated
by anybody, whereas releases are only published by python.org.
Each line has their own ProductVersion scheme: snapshots use
major.minor.currentday; official releases use the version from the
Python DLL. The relevant information is now extracted from
patchlevel.h.

Accordingly, there is no need to specify current_version anymore
in config.py, and the "alpha" setting has been renamed to "snapshot".

Regards,
Martin