Sample rate does not return correct sample rate for custom rate on simulator

[andrewjaykeller]

   OpenBCIBoard.prototype.sampleRate = function() {
        if(this.options.boardType === k.OBCIBoardDaisy) {
            return k.OBCISampleRate125;
        } else {
            return k.OBCISampleRate250;
        }
    };

If simulator is using custom sample rate this method should return it.

[andrewjaykeller]

This is addressed in #60