RtpTransceiver bindings & requirements for publishing tracks (#39)

- RtpSender
- RtpReceiver
- RtpTransceiver
- RtpParameters
- VideoFrameBuilder
- Interior mutability on c++ side
- Cleanup bindings ( Use #pragma once instead of include guards )
     - webrtc-sys/src/* headers are now used in only one place 
     - Avoid confusion between generated headers and our headers
- AdaptedVideoTrackSource
- Cleanup the workaround with unsupported Vec<Shared<T>>
    - Put everything inside one file

webrtc-sys/src/lib.rs

@@ -1,11 +1,14 @@
 pub mod candidate;
 pub mod data_channel;
+pub mod helper;
 pub mod jsep;
 pub mod media_stream;
 pub mod peer_connection;
 pub mod peer_connection_factory;
 pub mod rtc_error;
+pub mod rtp_parameters;
 pub mod rtp_receiver;
+pub mod rtp_sender;
 pub mod rtp_transceiver;
 pub mod video_frame;
 pub mod video_frame_buffer;