message: added maximumSpendable function to sdk to call new endpoint
  Reviewers: arik, benchan, zacleids, mark
  Subscribers: ben
diff: @@ -177,6 +177,40 @@ Wallet.prototype.transferBySequenceId = function(params, callback) {
  .nodeify(callback);
  };
  +/**
  + * Get the maximum amount you can spend in a single transaction
  + *
  + * @param params {Object} parameters object
  + * -walletPassphrase {String} the users wallet passphrase
  + * -limit {Number} maximum number of selectable unspents
  + * -minValue {Number} the minimum value of unspents to use
  + * -maxValue {Number} the maximum value of unspents to use
  + * -minHeight {Number} the minimum height of unspents on the block chain to use
  + * -target {Number} sum of the outputs plus sum of fees and change
  + * -plainTarget {Number} the sum of the outputs
  + * -minConfirms {Number} all selected unspents will have at least this many conformations
  + * -feeRate {Number} fee rate to use in calculation of maximum spendable
  + * @param callback
  + * @returns maxAmount {Number} the maximum amount you can send in a single transaction
  + */
  +Wallet.prototype.maximumSpendable = function maximumSpendable(params, callback) {
  + return co(function* () {
  + params = params || {};
  + common.validateParams(params, [], ['walletPassphrase', 'xprv'], callback);
  +
  + const filteredParams = _.pick(params, [
  + 'minValue', 'maxValue', 'minHeight', 'target', 'plainTarget',
  + 'limit', 'minConfirms', 'enforceMinConfirmsForChange', 'feeRate'
  + ]);
  +
  + return this.bitgo.get(this.url('/maximumSpendable'))
  + .send(filteredParams)
  + .result();
  +
  + }).call(this).asCallback(callback);
  +
  +};
  +
  /**
  * List the unspents for a given wallet
  * @param params
  