Save a step count sample.

A step count sample represents the number of steps during a specific period of time. A sample should be a precise as possible, with startDate and endDate representing the range of time the steps were taken in.

`saveSteps` accepts an options object containing required *`value: number`*, *`startDate: ISO8601Timestamp`*, and *`endDate: ISO8601Timestamp`*.
```javascript
// startDate and endDate are 30 minutes apart.
// this means the step count value occurred within those 30 minutes.
let options = {
  value: 100,
  startDate: (new Date(2016,6,2,6,0,0)).toISOString(),
  endDate: (new Date(2016,6,2,6,30,0)).toISOString()
};
```

```javascript
AppleHealthkit.saveSteps(options, (err, res) => {
  if (this._handleHKError(err, 'saveSteps')) {
    return;
  }
  // step count sample successfully saved
});
```