Reintroduce Alleged IDEA

All the patents on IDEA have now expired. The trademark on the name remains, hence "Alleged IDEA". This pull request is based on the original IDEA code from dlitz/pycrypto@v2.0.1 which was removed in dlitz/pycrypto@v2.1.0alpha1.

All tests pass as expected.
Let's not add a dependency on the sockets library just for a byte-swap.