Add basic Erlang-VM-style tracing utilities

This is a cousin of the https://github.com/basho/bitcask/issues/51 request over in the Bitcask repo.

Add basic Erlang VM tracing utilities.  Their source is in the `priv/tracers` directory.  The `rebar.config.script` file was edited to kludge Rebar into recognizing & compiling those files -- an entry in `Makefile` isn't sufficient while compiling Riak or RiakEE.