python: Have setup.py read the version number without importing

Importing the notmuch module in setup.py is a no-no, and we want
to auto-generate the version number in the release process.

Outsource __VERSION__ to the new version.py which contains nothing else
and which can therefor easily be autogenerated. Have setup.py read in
the file via execfile and test if importing the version number actually
worked.

This should make all happy.

Signed-off-by: Sebastian Spaeth <Sebastian@SSpaeth.de>