Localforage, browserify and IE8 do not play nicely together

When requiring localforage with browserify IE8 baulks with 'Object doesn't support this property or method' type errors. 

This is due to the fact that browserify will bundle up the `promise` module which is not es3 compatible. The compiled version (in the dist directory) uses a different promise library - `es6-promise` which _is_ es3 compatible. This PR contains a tweak so that browserify will also bring in `es6-promise`.

It's not a very elegant solution, in fact it feels a bit hacky, but I erred on the side of caution and tried not to touch anything.

Note that even with this change a browserified version of this package is still not quite IE8 ready. For that you need to filter out the es3 reserved keywords using something like [dekeywordify](https://github.com/pluma/dekeywordify)
Couldn't this go [here](https://github.com/mozilla/localForage/blob/master/src/localforage.js#L5) instead of in its own file?