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.