Create Sessions API "DELETE /sessions" test
Create DELETE 'api/v1/sessions' route
Change Sessions API "DELETE /sessions" test, no
need to post and log-in first
Created destroy action and authentication 
callback methods in API:V1:SessionsController, 
spec tests passing
Refactor API::V1::SessionsController to inherit
from API::V1::ApiController, screw devise sessions
and warden. Created API::V1:ApiController and
moved authentication token check logic there.
Make sure authentication tokens for users are
saved as strings. All tests passing.