Auto-sniff for AMD when moduleType is unknown.

In-progress fix for #45.

* Factored-out the code finder from findRequires and reused it to look for evidence of AMD in a module.
* Look for AMD evidence when module type is unknown when instantiating a module.
* Stop inferring the moduleType when loading metadata since we're inferring in the instantiate Loader hook now.
* Always pick "amd" or "globals" when inferring.

Initial smoke tests look good.  Still needs more unit tests.