[WIP] 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.