[MRG] Do not import the sklearn package from setup.py

The execution of setup.py never imports the sklearn package to avoid cyclid
dependencies.

The sklearn/version.py file is now generated each time setup.py is executed.
If the version is not a release version, a '.dev_aaaaaaa' suffix is appended
where 'aaaaaaa' is the 7 first digits of the hexdigest of the last git
commit in the local repo.

I find this solution less hackish and more informative than the previous
stateful builtins-based hack.