Make XMPPConnection an interface

create AbstractXMPPConnection.
Improve parsing. Add method for de-serializing Strings

Move some duplicate code from XMPP(TCP|BOSH)Connection to
PacketParserUtils. Remove TestUtils as the method now part of Smack's
public API in PacketParserUtils.
Remove schedule() from XMPPConnection interface

The idea that we abstract the scheduling of tasks on Android over this
method turned out to be unnecessary. schedule() was also not really part
of the *public* XMPPConnection API, so it's good that it's gone.
Make PacketReader and PacketWriter nested classes

of XMPPTCPConnection.
Add .travis.yml