The header consisted of too many unrelated stuff and macros misuse.
Some stuff is moved into scram.hrl and type_compat.hrl.
All macros have been replaced with the corresponding function calls.
TODO: probably type_compat.hrl is not even needed anymore since
we support only Erlang >= OTP 17.5
The dictionary returned after the directory call contains a meta key
whose value is a JSON dictionary. This is now taken care so that only
bitstring values are kept as resource URIs
1. A communications module that handles all requets/responses and other low level stuff that have to do with the ACME CA
2. A head module that will do all the useful stuff
