Exclude tests package from setup.py.

Pull request nvie/rq#192 solved the issue for `pip install rq` but not when
directly installing from a git repository as in
`pip install git+git://github.com/nvie/rq@master` that still creates a
possibly conflicting `tests` folder.
Merge pull request #280 from blaze33/master

Exclude tests package from setup.py.