message: Add gasPrice parameter to wallet.prebuildTransaction
  Summary: The server stuff is done, but SDK users need to be able to pass gasPrice into prebuildTransaction (via sendMany)
  Reviewers: barath
  Subscribers: ben
diff: @@ -731,7 +731,7 @@ Wallet.prototype.prebuildTransaction = function(params, callback) {
  'recipients', 'numBlocks', 'feeRate', 'minConfirms',
  'enforceMinConfirmsForChange', 'targetWalletUnspents',
  'message', 'minValue', 'maxValue', 'sequenceId',
  - 'lastLedgerSequence', 'ledgerSequenceDelta'
  + 'lastLedgerSequence', 'ledgerSequenceDelta', 'gasPrice'
  ]);
  const self = this;
  