A function that, when called, produces a Promise that resolves a specified amount of milliseconds since a particular timestamp - or immediately, if the delay since that timestamp has already passed.
This is particularly useful when implementing rate-limiting and/or task distribution code - your implementation only has to track the timestamp of the last call, and leave it up to this module to decide when to resolve a Promise that kickstarts the next task.
## 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.
Creates a new Promise that will resolve `delay` milliseconds from the specified `sinceTimestamp`. If the current time is later than `sinceTimestamp + delay`, the returned Promise will resolve immediately.
- __sinceTimestamp:__ The reference timestamp to start counting from, as a 'UNIX timestamp' in milliseconds (like you get from `Date.now()`).
- __delay:__ The delay as a number, in milliseconds.