Superficial chracter changes

To make pull request understandable, extract superficial
character changes in src/ directory at first.

Changes included are as follows:

RiakcPid -> RcPid
RiakPid  -> RcPid
Riak     -> RcPid
Pid      -> RcPid
Riakc    -> RcPid

pid()    -> riak_client()
Introduce riak_cs_riak_client and riak_cs_pbc

riak_cs_riak_client is a PB connection wrapper.
It has CS data type aware interfaces such as get_bucket.

Also low level PB functions are moved from riak_cs_utils to
riak_cs_pbc.
Change connection/pool usage

From riak_cs_riakc_pool_worker to riak_cs_riak_client.

- checkout from pool
- checkin to pool
- start/stop standalone processes
Use underlying PB connections in riak_cs_riak_client
Include multibag related process to supervisor tree
Treat Bag ID in buckets and manifests objects

- Assign bag IDs to buckets and manifests (if needed).
- Extract bag IDs from buckts and manifests and use
  them to retrieve their manifests and blocks (if needed)
- Add script to manipulate multibag configuration
GC with multibag

Also refactor out key listing logic to riak_cs_gc_key_list module.
Storage calculation with multibag
Fix premature connection release

WM resource produces body by know_length_stream, its finalize_request
is called immediately after it returns know_length_stream.
Releasing connection at finalize_request is premature because it is
used in producing body.
Bucket cleanup with multibag
Fix bug that no worker can not be checked out
Cosmetics: white space changes, tiny refactoring
Clean up unit tests
Clean up riak_test
Fix up build tools
Fix some type definitions
Avoid including riak_cs.hrl
Change riak_cs_multibag's branch to develop
Merge pull request #874 from basho/feature/riak-client-rebased

Riak client abstraction [rebased1]

Reviewed-by: kuenishi