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!
Good points.

So, the RFC says `mpint` is a `string` and a `string` is a uint32 length followed by that number of bytes. What do you think about creating a `_read_str_list` function that parses all of the string data out of the input and returns back a list of the data partitions of each string? It would then be very easy to check the length of the list returned to verify the expected number of strings is correct and also throw exceptions if the lengths don't add up to the size of the data.

In addition, the existing `_bytes_to_int` could be renamed to `_mpint_data_to_int` with no additional changes.