added batched authorizeSpend calls.

When the number of accounts grows, the SlaveBanker ends up initiating a large amount of REST requests to authorize further spend and report the spend of each account individually. The goal of this branch is to batch these requests under a single call thus reducing the overall communication overhead.
Moved the arguments intot he SlaveBankerArguments class which reduces to duplicated code but does have one very ugly hack. Oh well.