Allow specifying exponential factor

Exponential strategy is no longer hard coded to be * 2.
Let's make the factor a private member, i.e. this.factor_. Let's also check that's it's greater than 1, otherwise there won't be any backoff.

```javascript
// Note that undefined == null.
if (options && options.factor != null) {
    precond.checkArgument(options.factor > 1, 
        'Exponential factor should be greater than 1 but got %s.',
        options.factor);
    this.factor_ = options.factor;
} else {
    this.factor_ = ExponentialBackoffStrategy.DEFAULT_FACTOR;
}
```

Note that the code is indented using 4 spaces, not 2.