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!
That works for me. 

If that's the case, should I still move the functions to `cryptography.utils` or just leave them where they're at? Personally, I think we should leave them where they're at since they are only used by this function but then we can revisit moving them later when we add ECDSA support since I believe it may require the ability to read negative mpints.