Clarify bin_or_sbin semantics

Erlang's reltool always creates a `bin` directory when creating
a rel.  So packages using reltool and node_package should only
have binaries in `rel/bin`.
Change hardcoded sbin paths
Merge pull request #34 from basho/jem-change-sbin-bin-semantics

Clarify bin_or_sbin semantics