Add support for Serverless Messaging (XEP-174)

LLService, an abstract class, handles registering a local link service and
serves as the primary point of interaction for client developers.
JmDNSService is an LLService implementation which broadcasts the local link service
via Multicast DNS using the JmDNS library.

Some notable new classes employed by LLService:

LLServiceDiscoveryManager, a child class of ServiceDiscoveryManager,
connects local link Presences discovered by LLService to the
EntityCapsManager. This is needed because traditional presence packets
are not exchanged in a local link configuration.

XMPPLLConnection, a child class of XMPPTCPConnection, handles the
incoming/outgoing connection distinction and automatically establishes
a new XMPP stream with each peer connection.