Batch addLabels support

Hi,

Many thanks for this excellent library, is impressive the pattern used on it, i like it a lot.

Today i was implementing in my code the batch-mode and i saw that addLabels doent works under it. So this is my pr:

- Implemented addLabels with REST calls instead of Cypher query
- Implemented addLabels command for batch-mode
- Added isBatchStarted to Client, used to avoid ask for hasCapability in addLabels

I added all the unit test required and i test it in real implementation.

I hope you like it. If you need any change just tell me.

Regards and thanks