Initial XEP-0198 support

What is done:
* [Basic acking scenario](http://xmpp.org/extensions/xep-0198.html#scenarios-basic) - acks and error handling
* [Resumption](http://xmpp.org/extensions/xep-0198.html#resumption)
* XMPPStreamManagement module (but most logic done in XMPPStream.m, is it possible to move something in extension module?)
* XMPPRoster module modified not to reset roster on disconnect when stream management enabled
* Session data (Session ID and packet counters) saved and restored from NSUserDefaults (Will it make problems with multiple XMPPStreams application?)

TODO:
* [Efficient acking scenario](http://xmpp.org/extensions/xep-0198.html#scenarios-efficient)