* dtls/err.py: Added error code ERR_WRONG_VERSION_NUMBER
* dtls/openssl.py: Added DTLS_server_method(), DTLSv1_2_server_method() and DTLSv1_2_client_method()
* dtls/patch.py: Default protocol DTLS for ssl.wrap_socket() and ssl.SSLSocket()
* dtls/sslconnection.py:
- Introduced PROTOCOL_DTLSv1_2 and PROTOCOL_DTLS (the latter one is a synonym for the "higher" version)
- Updated _init_client() and _init_server() with the new protocol methods
- Default protocol DTLS for SSLConnection()
- Return on ERR_WRONG_VERSION_NUMBER if client and server cannot agree on protocol version
* dtls/test/unit.py:
- Extended test_get_server_certificate() to iterate over the different protocol combinations
- Extended test_protocol_dtlsv1() to try the different protocol combinations between client and server