Add AMD support

[ghost]

Add support for using es6-promise-polyfill as a AMD module.

Note: I couldn't create a new promise.min.js because there are no instructions on how to do so (See #7).

[lahmatiy]

AMD is already supported. Use something like this:

define(["./path/to/promise.min.js"], function(Promise) {
    var promise = new Promise();
});

[ghost]

A bit of a misconfusion here: AMD is indeed supported but it doesn't work in browser (Using RequireJS). See #9 .