pip-installable, include libsodium-0.4.2 tarball, remove need for system-wide libsodium

This branch changes setup.py and the use of FFI to use an included copy of libsodium-0.4.2 (as a tarball), rather than depending upon a system-wide libsodium.so (installed to /usr/local/lib or similar).

With this, you can do "pip install pynacl" and (if you have "tar" and "make" in addition to python) the result should just work, without a separate download/install of libsodium.

It uses `ffi.dlopen()` rather than `ffi.verify()`, which is not a trivial change. I changed the way the wrapper is built in nacl/nacl.py to match. I *think* it should have the same functionality as before, and the test suite passes, but I'd appreciate a second opinion.

Upgrading to a new libsodium should be easy: replace the tarball, and modify the filenames in setup.py.

This should close #28.