Add AMD support

I wasn't able to test, since I'm getting the following warning on `grunt test`, even on a fresh repo, and I'm not sure how to resolve it:

```bash
Running "clean:test" (clean) task
Cleaning "tmp"...OK

Running "emblem:ember" (emblem) task
File "tmp/emblem-ember.js" created.

Running "emblem:basic" (emblem) task
File "tmp/emblem-basic.js" created.

Running "simplemocha:all" (simplemocha) task
Warning: Arguments to path.resolve must be strings Use --force to continue.

Aborted due to warnings.
```
Running `npm test` doesn't give me any helpful clues either, unfortunately.
But it works for me as expected in a test run of `grunt emblem:compile` with the option `amd: true`.