Add support for IAX2

[Stickman]

It would be great if you could add support for IAX2 protocol in
Sipdroid.

I suppose you all already know IAX2 and the benefits it brings to the
VoIP scene, mainly to people with Asterisk servers.

There is a couple of pure java implementations that could be used to
bring IAX2 support to Sipdroid.

JavaIAXClient
http://sourceforge.net/scm/?type=svn&group_id=251710

and njiax

http://code.google.com/p/njiax/

I don't have the skills to do it, but maybe some of you will take some
interest in it.