new ValGens + riak_dt+pb driver(s)

[zeeshanlakhani]

Note: This needs another round of runs to make sure it's still working, as I've not used it in some months. Also, it could use a bit of cleanup as @russelldb and I were working through some ideas :). Nonetheless, tons of good work here that we don't want to lose; e.g. so many new, interesting, variable generators!

• Add a fixed size binary set generator (from rdb's bigset work)

  For testing CRDT sets we often want to generate a set of elements of
  elements of a certain size. In the past we used int generators, but
  sets of 32bit elements aren't realistic enough. We want to know how
  many elements of size X before performance degrades. To that end this
  commit adds a generator {fixed_bin_set, ElemSize, Cardinality} where
  ElemSize is the size of a random binary element and Cardinality is
  the size of the set. It makes use of the existing binary block
  generation code in valgen.

  (cherry picked from commit 3471aaf)

• Add poisson_bin valgen w/ workaround to approx poisson via normal dist
  when over a threshold size (cause it could be too slow!)

• Add variable size binary set generator to create bins within
  (minsize/maxsize, poisson dist, exp dist) params over a specified
  cardinality

• basho_bench driver for riak datatypes over pb, focused on sets right
  now for current benchmark needs (and comparisons to bigsets)

• driver allows for benches over preloaded sets, a single set, and
  normal-stylings (creating sets via keygen) and contains calls for
  batch_inserts via rdb's work

• new distributions/valgens for bins and gen'ing/using them w/ var_bin_sets

• cleaner examples

• add maps benchies & examples

• add more sets bench operations & examples