Added RPC call 'getmemorypool' that provides everything needed to construct a block...

Added RPC call 'getmemorypool' that provides everything needed to construct a block with a custom generation transaction and submit a solution

getmemorypool [data]
If [data] is not specified, returns data needed to construct a block to work on:
  "version" : block version
  "previousblockhash" : hash of current highest block
  "transactions" : contents of non-coinbase transactions that should be included in the next block
  "coinbasevalue" : maximum allowable input to coinbase transaction, including the generation award and transaction fees
  "time" : timestamp appropriate for next block
  "bits" : compressed target of next block
If [data] is specified, tries to solve the block and returns true if it was successful.