XEP-0079: Advanced Message Processing

This pull request consists of an implementation of (a subset of) [XEP-0079](http://xmpp.org/extensions/xep-0079.html).

Differences from the standard are documented in ` doc/MongooseIM/mod_amp_xep-0079.md`
 `doc/MongooseIM/mod_amp_developers_guide.md`

Unit tests can be found in `apps/ejabberd/tests/*`, while escalus acceptance tests reside in the [ejabberd_tests](https://github.com/esl/ejabberd_tests) repo, under `amp_SUITE.erl`.