Allow RiakBucket.new to return datatypes.
Add type name to each datatype class and centralize the TYPES mapping.
Cleanup codec of datatypes req/resp over PBC.

- Unify request options settings into a single method.
- Unify extracting value so it will work with both fetch and
  update+return_body.
- Make use of the datatype's "type_name" property instead of looking
  things up or checking isinstance().
Add resource URL generator for datatypes.
Fix typo and PEP8 warning
HTTP datatypes now working.