Refactoring: All connection classes begin with XMPP now

This commit renames classes as follows:
 * TCPConnection --> XMPPTCPConnection
 * BOSHConnection --> XMPPBOSHConnection

There are two reasons for this rename. First, it is there to indicate
that the classes actually _are_ XMPP connections, using different
transport mechanisms. Second, it makes auto-completion in IDEs easier,
the developer can type XMPP<complete> and choose the right backend.