Last night modifications

- Created a `Mongovel\Cursor` class that wraps `MongoCursor` while extending Illuminate's `Collection` class — the latter implements interesting interfaces such as `JsonSerializable` which per example allows you to directly do things like `return Response::json(Document::find())`. This will take into account both the Cursor and the models's `toArray` method, taking into account hidden fields and such.
- I added support for the aformentioned hidden fields on models
- I made the `update` method use `handleParameters` so you can now do `Document::update('documentId', array())
- Fixed some bugs and added comments where needed
- Added some quick tests

[Diff without whitespace](https://github.com/julien-c/mongovel/pull/1/files?w=1)