TLS certificate verification (thanks Rohit Agrawal, Klaus Darilion, and Pierre-Luc Bacon for the suggestions)

[nanang]

Issues:

• Domain name verification, IETF RFC draft could be found ​here.
• Add callback in PJSUA-LIB to notify transport state (e.g. transport is connected or disconnected). There are several use cases for this:
  • application send re-REGISTER when the connection is down. This should reconnect the transport.
  • application can inspect TLS connection/verification error and reconfigure TLS transport respectively (for example, to disable domain verification if TLS connection fails due to certificate error (perhaps after asking for user confirmation))