added post callback hook as behaviour
fixed hook to work for check_token/code, resolves #2
fixes so that erlang.mk compile works

erlang.mk has `-Werror` on by default, so the warnings
were breaking the build, and relx was giving errors for
.app.src.
Merge pull request #4 from jtmoulia/no-warnings

fixes so that erlang.mk compile works
Merge pull request #3 from jtmoulia/callback-hook

Callback hook introduced