Ping improvements

[white-gecko]

Additionally to #40 following things should be improved for the Erfurt_Ping class:

• receive() should return true or false
• an additional method getStatusCode()
• an additional method getTriples() which returns the new data from the source
• receive() should not always write the received triples to the store
• to make the pingback service usable for Erfurt standalone we have to move parts of linkeddataserver plugin to erfurt
• specify target graph for the ping to go in options
• add config option to also trigger onPingReceivedAgain event (onPingReceived event only triggered first time OntoWiki#90)

[white-gecko]

Done:

• to make the pingback service usable for Erfurt standalone we have to move parts of linkeddataserver plugin to erfurt

[white-gecko]

Created new branch for this issue: https://github.com/AKSW/Erfurt/tree/feature/pingback-improved

Done:

• receive() should return true or false
• an additional method getStatusCode() -> getReturnValue()

[white-gecko]

Done:

• an additional method getTriples() which returns the new data from the source -> getReceivedData()
• receive() should not always write the received triples to the store -> set $option['write_data'] = false