fix issues in "old" pingback
change pingback to use model for storing pings
Add PingTest and add methods for injection to Ping
Add Memory Store Adapter and PingTests
Mark tests as incomplete to prepare it for merge

The test have to be skipped because of an authentication problem in
versioning.
move return value to extra method
add protected _getResource method
move determineInverseProperty method from Ping to Utils
add config option to not write data immediately

and remove determineInverseProperty method (-> Util)