Move processPacket() method to XMPPConnection

This removes code duplication in BOSHConnection and TCPConnection's
PacketReader, by moving the common code to the superclass
XMPPConnection.
Introduce SmackException

SmackException (and it's subclasses) is for all errors/exceptions not
defined by any XMPP specification. XMPPException is now an abstract
class for all errors defined by the XMPP specifications.

Methods that involve an IQ exchange now either return the result, which
is obtained by IQ response, or they throw an XMPPErrorException if an IQ
error was the result of the IQ set/get. If there was no response from
the server within the default packet timeout, a NoResponseException will
be thrown.

XMPP SASL errors are now also reported accordingly.

SMACK-426
Move getRoster() to XMPPConnection

Also remove the Exceptions from the signature of getRoster().