Solved when JSON returned is empty

When there is no elements requested the JSON returned is empty,
in this case the extract function in the RESTAdapter must return
a empty array.
Merge branch 'wip/ember_migration' of https://github.com/MateusZitelli/traintool into wip/ember_migration

Conflicts:
	frontend/app/adapters/traintool_rest.js
Test routines working

Until the moment there is only the test to verify if a
exercise is created correctly. To isolate the tests
from the REST api a clone script from the app is generated
with a localstorage adapter instead of the RESTAdapter.
Then this other version is added for the qunit test.