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!
Another thought: The simplest solution here might be to rename `_read_next_mpint` to `_read_next_pos_mpint` and fix the negative numbers when we move on to other key types since its not necessary for RSA. (Just brainstorming here)