Besides the way the transport handles the stream after SASL
<success/>, the SASL logic is independend from the underlying
transport (BOSH, TCP, …). Hence move it up into
AbstractXMPPConnection.
This also has the benefit that we can make some more methods private
or package-private.
Also introduce XmlStringBuilder.optTextChild(), which causes some
associated changes.
Introducing Smack's own XmlPullParser interface which tries to stay as
compatible as possible to XPP3. The interface is used to either wrap
StAX's XMLStreamReader if Smack is used on Java SE, and XPP3's
XmlPullParser if Smack is used on on Android.
Fixes SMACK-591.
Also introduce JUnit 5 and non-strict javadoc projects.
