Added support for Basic Authentication

RTSP streams protected by Basic authentication were failing to authorize.

`RTSP::Message` parses the URI for user/password and sends the Authorization header as part of the default headers.

`RTSP::Client` parses the URI for user/password and allows for directly setting the user/password values. When a 401 response is encountered, the request message will be repeated with the Authorization header set.

I tested with a Dropcam RTSP stream but the SDP(0.2.9) gem is currently raising a `SDP::ParseError` when encountering "a=control:". This appears to be a known issue with the SDP gem. https://github.com/turboladen/sdp/issues/24