Don’t rely on `Function.prototype.bind`

Hi Christoph, thank you for creating ayepromise. I found it by searching for a minimal implementation.

Consider merging this pull request, please. It removes the need for a `Function.prototype.bind` polyfill and thus makes integration of this library much more convenient when working with legacy browsers (or iOS). 

Maybe *one* use of `bind` can also be done in a “traditional” way instead of requiring a polyfill ;-)

Cheers, David