Update python packaging

Added a requirements.txt file for easier dependency management.
Updated setup.py to automatically create the mp and mailpile scripts.

This should make developing mailpile bit easier, I think. Instead of creating the mp script (or mailpile) in the source root it's placed in /usr/local/bin or <venv>/bin/ (when using virtualenv) which makes running them a lot easier and no mucking with PATH or PYTHONPATH or having to run the script from the source root.
When developing instead of running `python setup.py install` use `python setup.py develop` which will add the source root path to the python search path (using <python_lib_path>/site-packages/easy-install.pth).