Added load_ssh_public_key to load OpenSSH public keys (RSA support only)

As far as I can tell, there isn't currently a way to import OpenSSH (RFC 4253) formatted RSA public keys into an RSAPublicKey in cryptography. This commit adds that functionality.

Other Python cryptography libraries (notably PyCrypto) provide this sort of functionality and at least one library that I'd like to see ported to cryptography relies on that functionality.

This is my first-time contributing to cryptography. Your feedback would be very much appreciated! Thanks!
I've got a set of SSH-related encoding and decoding functions that you might want to take a look at in:

https://github.com/ronf/asyncssh/blob/master/asyncssh/packet.py

Converting an MPInt to an int isn't all that hard if that's all you need to do, but converting the other way can be a bit tricky due to issues around choosing the right length. Since the encoding is for signed integers, some unsigned values need to include an extra byte so they are not interpreted as signed.