Update for R15+ typed callbacks

Hello,
I made two small updates to elli.hrl and elli_handler.erl, using -callback (R15+)
to make dialyzer happy if you create and check an elli handler module.
Without the patch dialyzer will complain: "Callback info about the elli_handler behaviour is not available". (tested with R16B)
Please pull it to your repo.
Thanks.