this is the first stop towards fixing "SMACK-65: parsing should look for
depth", by providing the initial parsing depth to the provider. Some
methods (.e.g parseMessage) now use the depth as abort condition,
instead of a unclean String equals check.
parseIQ() and parseExtension() where both renamed to parse.
This also restricts the Exceptions thrown by the parse method, to just
XmlPullParserException, IOException and SmackException (not really a big
victory, but nevertheless a slight improvement).
StreamFeatureProvider is now gone, we simply use PacketExtensionProvider
for stream features.
