Add a stats/1 callback to all datatypes.

The callback returns a proplist of atom/number pairs about internal
statistics of the datatype. Examples include 'actor_count' (on any
VV-based type), 'element_count' (sets), 'field_count' (maps),
'max_dot_length' (on dotted VV-based types).
Add behaivour and stats callback to some datatypes that didn't have it.
Update specs on stats callback and other public functions.
Merge pull request #69 from basho/feature/sdc/stats

Add a stats/1 callback to all datatypes.