Add examples related to Text Messages

[Naapperas]

Since this is a feature that exists, it should be documented. One example of how to deal with text messages using this wrapper (receiving/sending/both) would be great.

[Ulexus]

I've never actually tried using it. I've never had an application for Asterisk's messaging system. We implemented it simply because it is part of the ARI specification.

Please feel free to try it out and PR and example, though.

[Naapperas]

I tried using it but never actually got it working. SIP MESSAGEs were getting fed into my system as a StasisStart event on the special "message" channel (I can't recall the correct name). Perhaps that was due to the way I configured my pjsip.conf file but was the only way I managed to receive messages. If I eventually manage to figure out how to do this with the wrapper I'll submit a PR.