Separate version string from setup.py

This is also a plea for publishing the newest package on PyPI :smile: 

I installed inbox from PyPI.  I followed the `README` and setup a simple handler but it errored it complained that the handler takes an extra `subject` parameter which was not passed from the base.  Looking at the changelog, it appears https://github.com/kennethreitz/inbox.py/commit/4b1c76955f798601c3f7d43bf2ad51521e731bc7#diff-d41d8cd98f00b204e9800998ecf8427e which added the `subject` parameter was newer than the version published on PyPI.