Fixes a bug in EPDB.Storage#GET(id) 

Fixes a bug in `EPDB.Storage#GET(id)` that caused the returned model to have a property `_id` instead of `id` and `_rev` instead of `rev`.

Thus, these models could not be updated through `EPDB.Storage#PUT(model)` method. To be more precise, as pouchdb could not find the id it saved the model as a new document.

Also modified the `storage_test.js` "POST & GET" test case to test for id + rev properties.