Eliminates unnecessary `src/` directory from project structure.

Also eliminates unnecessary manipulation of PYTHONPATH, since
`pip install -e` already creates a link to `chatexchange` in the
environment's site packages.
Merge pull request #33 from jeremybanks/no-src-directory

Eliminates unnecessary `src/` directory from project structure.
Merge pull request #34 from Manishearth/test

Forward test to master