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!
Actually, this wouldn't work because SSH public keys are allowed to have a comment after the type and certificate body (key_parts[2]). If we reduced this block to your suggestion, the current tests would fail because the comment would be included as part of the key_body which is not correct.