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'd like this logic factored out into a `_read_mpint` function, which returns `(int, rest)` (rest being the remainder of the bytes). I think that will make this logic easier to follow and more obviously correct.

Right now this code will silently accept public keys with a bunch of trailing data; both `data.split(b' ')` and this bit don't complain about extra stuff. I think both cases should raise.