Add a test for the filename regression

Files that have the same filename in a different path step on eachother
and prevent one or the other from being properly loaded.

Unfortunately, i can't think of a solution that doesn't require setting some sort of base path option and then requiring the filename in the `map` be relative to that path. E.g. 

```coffeescript
emport 'app.coffee', root: __dirname, paths: [ 'modules', 'frontend' ], map:
  'frontend/vendor/jquery.js'      : exports: '$'
  'frontend/vendor/underscore.js'  : exports: '_'
  'frontend/vendor/backbone.js'    : exports: 'Backbone', imports: ['_', '$']
```