Add support for 'baseUrl' option

Useful when the baseUrl of the AMD when running tests would be
different from the baseUrl when running dev/production code.
See [this example](https://github.com/jodal/buster-coffee/blob/master/demo-amd/test/require-config.js).
Merge pull request #1 from vitorbal/master

Add support for 'baseUrl' option