Added key exchange method "diffie-hellman-group14-sha1".

I lately failed to use paramiko to connect to an SSH server which does not implement [diffie-hellman-group1-sha1](http://tools.ietf.org/html/rfc4253#section-8.1) which actually violates the RFC 4253.

OTOH paramiko violates the same RFC in that it does not support [diffie-hellman-group14-sha1](http://tools.ietf.org/html/rfc4253#section-8.2), which must be implemented all the same.

Please pull this branch to add support for this kex protocol to paramiko.