Add RangeReg as a new supported datatype

RangeReg is a register for integers that tracks the largest,
the smallest, the first (by timestamp) and the last (by timestamp)
integers assigned into it.

The new type is called "rangereg" inside the codebase, and can be
activated just like any other bucket-type datatype. It exposes
the exact same statistics as the existing 2.0 datatypes.

This requires the work in basho/riak_dt#59 to be merged before it can
be merged. I'm also about to open a lot of sub-pull-requests for riak_pb, 
riak_test and several other repos, all of which go along with this PR.

It's worth saying this is 2.x (where x > 0) work, so not immediate.
This could do with some docs. Oops