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 #680 from basho/feature/object-limits

Add warning/max object limits