chan_rtp: Add unicast RTP support.

This module supports sending both unicast and multicast RTP
to a specified target. Multicast functionality is the same as
chan_multicast_rtp was. In the case of unicast a specific
IP address and port can be specified, along with optional RTP
engine and format in the form of:

UnicastRTP/<ip address>:<port>/<engine>/<format>

This can be useful for sending a copy of a media stream to
another application for processing.

Review: https://reviewboard.asterisk.org/r/3981/


git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@423004 f38db490-d61c-443f-a65b-d21fe96a405b