ES6 module rewriting needs configurable base path

As discussed here:
https://groups.google.com/d/msg/closure-compiler-discuss/Lh890LsQ8FY/z4nohpYzfdwJ

The standard for other module systems which ES6 is/will probably take is:
- `"path/to/Module"` => absolute path against a base URL, typically html document in browser, and you don't need ".js", it will be added, i.e. `"BASE/path/to/Module.js"`
- `"./OtherModule"` => `"OtherModule.js"` in same folder as importing script
- `"../OtherModule"` => `"OtherModule.js"` in parent folder of importing script

Currently ES6ModuleLoader.NaiveModuleLoader uses JVM current working directory as base for absolute module paths, which isn't always appropriate.