adding isomorphic support

I really know this spec is for browser.
but I believe make it isomorphic works fine for developer.
if I use this API on both browser and server.
it will help boost javascript isomorphic.
ex) 
- sharing logic in node.js.
- running test on cli without heavy headless browser.

This will mean that bluebird's code will _always_ be added to this file when it's browserified and delivered to browsers that already support Promises…  This will make the compiled code a lot bigger — not sure you want to do it like this…  I've written an alternative suggestion in the comments.