code: function(key, increment) {
          this._removeRemove(this.counters.remove, key);
          var op = this._getOp(this.counters.increment, key);
          if (op) {
              op.increment += increment;
          } else {
              this.counters.increment.push({key: key, increment: increment});
          }
          return this;
      }
docs: Increment (and/or create) a counter inside the map.
  @method incrementCounter
  @param {String} key the key in the map for this counter.
  @param {Number} increment the amount to increment (or decrement if negative)
  @chainable