Wrote test TestPayload which will check that we can correctly get the payload from an RTP
packet using Payload for a variety of different RTP packet configurations.
Wrote test TestHasExt which checks the behaviour of hasExt for when it's call with an
RTP packet with an extension indicator and also for an RTP packet with no extension
indicator.
Added parse.go. This file contains functionality to obtain the Payload from an RTP packet.
Unexported functions to help with this extraction have been added; extHeaderLen, hasExt,
csrcCount and version.
Much of the functionality we require for rtsp input is here, but some modification will be required to fix some bugs and get it functional; which will come in later PRs.