Added message signing

Added message signing and test function. Private keys are used in their Base58 import format, as such they are also not stored in memory very securely. However, if you are going to use libraries such as this you should already have a secure system to work on.

I indent to further this by creating a python c extension, pure python implementation of message signing/verification currently requires 600ms+ per signature. This code can take that down to ~800us. An improvement of nearly 1000x.

I indent to fork for the python extension as it would be quite a different structure to what you have here. But i'll give you credit. I haven't tested testnet signing or done any massive amounts of unit tests, so that's something that could be done.

Thanks