Add getLastStanzaReceived() to XMPPConnection

also remove faulty PongFilter from PingManager. It never matched any
stanzas, since a Pong is just a plain result IQ that is not qualified by
any XMPP Ping namespace.

Fixes SMACK-597
Fix PingServerRunnable scheduling

should be nextPingIn and not pingInterval.
Make the next server ping code easier to read
Updated providers.md: s/.getInstance()//

As ProviderManager has now only static methods
Make parseElementText handle empty string elements

ie. elements which only content is the empty string, for example
<body></body>.

Fixes SMACK-598
Merge branch '4.0'