Use lager:debug instead of io:format.

Using lager allows to easily switch out the logging level in the
application which uses gen_smtp as well as providing a much richer set
of debugging tools.

Closes #51.
Make gen_smtp dialyze, bumping it up to modern day Erlang versions.
Remove the compatibilty shims from the binstr module.
Merge commit -_-.
Update SSL certificate/key.