Several misc. setup tasks

- Added setuptools bootstrapping and setup.py file so popgen can be installed as a library (called popgen)
- Cleaned up imports in each file (organized them and removed those that are unused)
- Made python code conform to [PEP8](http://legacy.python.org/dev/peps/pep-0008/)
- Added Travis-CI integration. See Travis at https://travis-ci.org/synthicity/popgen and coveralls at https://coveralls.io/r/synthicity/popgen
- Added contributor guidelines in CONTRIBUTING.md