Add additional configuration items to basho_bench_driver_riakc_pb.erl

* pb_connect_options, list(two_tuple()), list of valid Protocol Buffers
  client connection options.
  Default = [{auto_reconnect, true}]

NOTE: The new default value creates a change in behavior of the
      PB client: the default is {auto_reconnect, false}.  Users
      of the PB client that wish the old & brittle behavior of
      crashing on server-side errors, timeouts, etc. must provide
      explicit configuration now to disable auto_reconnect behavior.

* pb_timeout_general, integer(), overall single Protocol Buffer
  operation timeout value.
  Default = 62000
  Units = milliseconds

* pb_timeout_read, integer(), overall single Protocol Buffer
  read operation timeout value.
  Default = value of pb_timeout_general
  Units = milliseconds

* pb_timeout_write, integer(), overall single Protocol Buffer
  write operation timeout value.
  Default = value of pb_timeout_general
  Units = milliseconds

* pb_timeout_listkeys, integer(), overall single Protocol Buffer
  listkeys operation timeout value.
  Default = value of pb_timeout_general
  Units = milliseconds

* pb_timeout_mapreduce, integer(), overall single Protocol Buffer
  mapreduce operation timeout value.
  Default = value of pb_timeout_general
  Units = milliseconds