Importer

see #691

refactored big parts of the implementation of `Evaluator` to support asynchron behavior.
extraced `Importer` for the `@import` lookup and loading stuff (asynchron).
(`readFileSync` is now `readFile`, to not block the process)

All tests passes but I'm not sure about all things I've changed. A review would be great.

The result of the refactoring is that you can now replace the `@import` resolution and loading process with your own stuff.