Report a NotFoundError when index-fetching from an empty store
Define IndexedModel test class for an upcoming backbone-db test

A better thing to do might be to define all models and collections
in backbone-db's test suite and only override the db and sync
properties in the implementation tests.
Merge pull request #1 from mzabaluev/bogus-empty-fetch

Properly report a NotFoundError when findAll by index is done on an empty store