Implement support for res_pjsip

[lelutin]

I've learned this week that chan_sip is being slowly replaced by res_pjsip (which was added in asterisk 12).

See: https://wiki.asterisk.org/wiki/display/AST/Migrating+from+chan_sip+to+res_pjsip

This module needs to have some support for res_pjsip so that users can start transitioning.

The configuration for that new element is way more verbose though, so it'll look and smell different.