Add support for AMD-style exports

This adds support for AMD exports.

When a single string is provided as the 'exports' config value, the
corresponding variable is returned as the AMD module's value.

When an array is provided, AMD's special "exports" dependency is used
to export multiple values as in a module:

https://github.com/amdjs/amdjs-api/wiki/AMD#using-require-and-exports
Merge pull request #7 from jdbartlett/amd-exports

Add support for exports when using AMD