Added support for HOXT (XEP-0332)

This is initial impementation of XEP-0332 (SMACK-552) -
HTTP over XMPP transport.
Created extensions, providers and unit tests.
Two features are missing: jingle and sipub.
Merge pull request #3 from Tsyko/xep-0332

Initial implementation of "HTTP over XMPP transport" (XEP-0332)
Move file related methods to FileUtils
Renamed DefaultRosterStore to DirectoryRosterStore
Make DirectoryRosterStore use elements for whitespace content

If whitespace (e.g. \n \t) needs to be preserved, then XML elements
should be used instead of attributes. Since we do this now in
DirectoryRosterStore, there is also no longer a need for an
specialized StringUtils.escapeForXML().

Also introduce XmlStringBuilder, which should become the default way
to implement Packet.toXML() and the like.