Make AMD module id's optional

Here's a PR for #186.

Since I really think inserting module id's should be _opt-in_ rather than the default (i.e. anonymous modules should be the default output), I'd like to get your opinion on this first.

Then either I need to fix the tests, or the default setting for this feature.
Most specs say "id" (http://wiki.commonjs.org/wiki/Modules/AsynchronousDefinition, https://github.com/amdjs/amdjs-api/wiki/AMD). But I'm completely fine with "names", your call.