C library for PoW

I added a C library to the client in order to speed up cpu intensive tasks. At the moment it only implements the PoW, but other functions can be added easily.

On Unix, the client will try to build the library on startup. Windows user must compile it by themselves and place it under "src/pybitmessage.dll", or you can distribute precompiled binarys.
It should also work on Mac, but I haven't one to test with.

However, it will fall back to the python implemention if the library can't be used or is disabled with "disableclib = True" in keys.dat.

The sha512 implementation is taken from polarSSL. If there are license issues, it's no problem to pick another one.

I don't know if you accept this kind of patch at all, so feel free to reject it ;)