Move riak_core.proto from riak_core.

This protobuff definition is only used by KV. Whether we should
continue to use this encoding can be debated separately, but putting
the file in KV removes yet another dependency from Core. Third parties
who use Core can pull in protobuffs via riak_api or their own
applications. For KV, it is pulled as a dependency of riak_api.

/cc @Vagabond @evanmcc @jrwest
Add max/warn object size & sibling thresholds

Reading or writing objects whose binary representation is bigger than
warn_object_size will generate a warning in the logs identifying the
bucket/key and size for the object.

Putting an object whose binary size is greater than max_object_size will
fail.

We will now warn if writing more than warn_siblings and fail to put if
more than max_siblings.
Merge pull request #659 from basho/refactor/sdc-rm-protobuffs

Move riak_core.proto from riak_core.
Merge pull request #680 from basho/feature/object-limits

Add warning/max object limits
Fix include path.

This resolves compilation errors on develop/riak_kv.
Merge pull request #705 from basho/bugfix/csm/pb-compilation

Fix include path.
Add generated header to .gitignore
Remove riak_core dependency because pipe and api bring it in
Merge branch 'develop' into bugfix/bucket-types-mr-incompatibility

To fix broken riak_core_pb header thingie