A function for writing *one per N amount of time* rate-limiting implementations.
When called, it returns a 'delayer' function. This delayer function can be called repeatedly; every time it is called, it returns a Promise that resolves `delay` milliseconds further into the future. The first time it is called, it resolves immediately.
So for example, if you specify a `delay` of 2000 milliseconds, the Promise returned from the first call will resolve immediately; the Promise returned from the second call will resolve after 2 seconds, the Promise resolved from the third call will resolve after 4 seconds, and so on.
All of these times are relative to the first call you make - so in the above example, there are 2 seconds between every resolution:
```
0ms Call 1
0ms Resolve 1
2ms Call 2
3ms Call 3
5ms Call 4
2000ms Resolve 2
4000ms Resolve 3
6000ms Resolve 4
```
If you wait longer than the delay to call the delayer function again, the returned Promise will resolve immediately and that time becomes the new base time for future delays:
```
0ms Call 1
0ms Resolve 1
2ms Call 2
2000ms Resolve 2
... time passes, beyond the 2000ms delay ...
7400ms Call 3
7400ms Resolve 3
7402ms Call 4
9400ms Resolve 4
```
## Example
See also `example.js` for a runnable version. This example uses Bluebird for `Promise.try` (read [this article](http://cryto.net/~joepie91/blog/2016/05/11/what-is-promise-try-and-why-does-it-matter/) to understand why), but Bluebird is not required to use this module.