Don't limit _id to the mongoDB default one

We should be able to specify a different ID, such like a `"string"` or a `"number"` and paths should still working.
Keep tests passing but authorize more flexible IDs

Some tests should still passing (just like `Model handlers should fail athirdroute (user defined route)`).

Still, we may wish to have an ID which is not 24 characters. An incremental ID number for instance.
Merge pull request #41 from nicoespeon/master

Don't limit _id to the mongoDB default one