Refactor to support standard Multi-task configuration (Fix #6)

There is the multi-task support.

I also refactored some of the code to use the built-in Grunt file system helpers. It's way less verbose than the `fs` functions and help to keep the code focused on the business logic rather than the implementation details.

I added some configs to the `Gruntfile` to have a more complete "test" scheme as there's no unit test yet.

It will probably broke some precedent configurations as Multi-task use the `options` hash rather than plain keys; so it would require a minor bump of version before publishing.

Hope this helps!