Add AMD (asynchronous module definition) support

This pull request adds support for [AMD](https://github.com/amdjs/amdjs-api/wiki/AMD), as used by module loaders such as require.js (with fallback if a module loader is not present).

I see you have multiple files for multiple versions, which is a bit unorthodox as one usually keep one file and manage different releases via git tags.

Because of this, I'm not sure how or in which file I should make a pull request.

This pull request's change is in the 1.3 file.