Embedded Sodium

Uses distutils built in c library support to build libsodium prior to building PyNaCl. It might also be a good idea to provide some way to not use the bundled copy of libsodium, we also need to figure out what this means for Windows support.

This branch does *not* work with ``pip install -e ...`` or ``python setup.py develop``. In those cases it would currently be expected of the developer to have sodium installed globally.

This fixes #28, and fixes #26, and closes #27, and closes #29, and closes #30.