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!
@alex Yes, that is certainly the option I prefer and is easy enough to fix. 

When I saw https://github.com/pyca/cryptography/blob/master/tests/hazmat/primitives/test_serialization.py#L520 , I assumed that you guys had decided to do .encode() instead and didn't want to break with the convention.