Added support for request batches
Added support for request batches

* It is now possible to use the Batch feature as described in the
JSON-RPC spec.
  * You create a batch with `var batch = jsonrpc.batch()`.
  * You add requests with `batch.add(request).then(handler;`.
  * You send the batch with `batch.send()`.
* Added documentation, unit tests and an example.
* Thanks to GerritK for most of the work of this feature!

joostvunderink/angular-jsonrpc-client,joostvunderink/angular-jsonrpc-client