Make worker init asynchronous
Make basho_bench easier to launch from another app

Reduce assumptions about which dependencies are already running or not
to make basho_bench easier to embed in another application.

Add basho_bench:start/1 that starts basho_bench using the configuration
passed in as the function argument.
Add 'riakc_pb_seed' configuration parameter

This configuration option takes an Erlang node name as a value.
When provided, basho_bench will automatically determine the
appropriate PB client IP/port for all nodes in the cluster that
the seed node is a member of.