Update secondary index responses

2i and CS bucket fold responses have additional fields from the previous
2i implementation. 2i now has a `return_terms` option, which will change
the response, and both 2i and CS bucket fold have a `continuation`
binary that will be returned (or `undefined`). This change does a couple
things:

* puts the continuation into the `done` message for streaming responses
* encapsulates 2i buffered and streaming responses into a record
* changes the response tuple-wrapper for CS bucket fold from `objects`
  to `ok`.
* adds new types for the continuation and new records
Update rebar to 2.1.0pre

$ ./rebar -V
rebar 2.1.0-pre R14B04 20130226_211548 git 2.1.0-pre-50-ge36b9c1

This allows dialyzer riak_pb to be build with +debug and have dialyzer
successfully run.
Remove trailing whitespace