Implement key formats and export file format function #96

I'd like to be able to choose the format of the key Im generating, for example: ssh-keygen -m PEM`, but I can't see how to do it on the library.
I was also trying other types of rsa keys (Maybe the default format would be the one I'm looking for), and it seems that they don't work:

https://github.com/ParallelSSH/ssh-python/blob/master/ssh/keytypes.pyx#L53-L60

key_type = keytypes.RSAKey()
print(key_type)

Outputs:
ssh-rsa

key_type = keytypes.RSA1Key()
print(key_type)

Outputs:
unknown