Adding a new test helper RtpFileWriter and use it in RTPcat

This new helper class writes RTP packets to file in rtpdump format.
A unit test is also included.

The new test class is used while re-writing the test tool RTPcat.

BUG=2692
R=pbos@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/28099004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@7768 4adac7df-926f-26a2-2b94-8c16560cd09d