Clean up two security holes: link walking and arbitrary erlang MR

Link walking is deprecated in 2.0, so we won't make it work with
security. Also, Erlang mapreduce phases and inputs must now use modules
that reside in an add_paths directory in the config file. This is to
prevent arbitrary use of the Erlang standard library to do malicious
things.
Merge pull request #756 from basho/adt-security-holes

Clean up two security holes: link walking and arbitrary erlang MR