Move the cryptography package into a src/ subdirectory

Due to differences in how py.test determines which module to ``import`` the test suite actually runs against the cryptography which is in the *current* directory instead of the cryptography which is installed. The problem essentially boils down to when there is a tests/__init__.py then py.test adds the current directory to the front of the sys.path, causing it to take precedence over the installed location.

This means that running the tests relies on the implicit compile that CFFI does instead of testing against what people will actually be runnning, which is the module compiled by setup.py.

This should also cause the tests to run a little bit faster as something like ``tox -e py34`` currently compiles everything 3 times instead of only once like it should. This PR will drop this down to 2 times.