Ported snowflake client to work with pion/webrtc

Modified the snowflake client to use pion/webrtc as the webrtc library.
This involved a few small changes to match function signatures as well
as several larger ones:
- OnNegotiationNeeded is no longer supported, so CreateOffer and
SetLocalDescription have been moved to a go routine called after the
other peer connection callbacks are set
- We need our own deserialize/serialize functions
- We need to use a SettingEngine in order to access the
OnICEGatheringStateChange callback

client/lib/interfaces.go

@@ -52,5 +52,5 @@ type SocksConnector interface {
 // Interface for the Snowflake's transport. (Typically just webrtc.DataChannel)
 type SnowflakeDataChannel interface {
 	io.Closer
-	Send([]byte)
+	Send([]byte) error
 }