allow pip installation

[r00tat]

I had some troubles using webapp2_extras session with ferris3 and found gae-sessions. It's great for my purposes.

I'm using pip and a requirements.txt to keep other python modules up to date. Please add a setup.py, so it can be installed even easier.

i.e.:
requirements.txt:

git+https://github.com/dound/gae-sessions.git#egg=gaesessions

installation to the lib folder:

pip install -r requirements.txt -t lib/

This pull request is an example of how the setup.py could look like.