[reSIProcate] stun support in UdpTransport

[Fischl jason <jason.fischl@xxxxxxxxx>]

I've put together some preliminary stun support in the UdpTransport in
resip. In particular, if you implement
http://www.softarmor.com/wgdb/docs/draft-ietf-sip-outbound-00.txt you
will need support for this.

I will try and add similar support into the TcpTransport in future as well. 

The KeepAliveManager should also be updated to send stun packets
instead of CRLF. If no stun response is received, the KeepAliveManager
should probably determine that the server doesn't support stun and
start sending CRLF instead.

For now, I've copied the stun code into a subdirectory of resiprocate.
This should be made part of contrib at some point.

Jason