Add support for 2.0 Bucket Types

[dmitrizagidulin]

Add an optional 'bucket_type' parameter to bucket creation. See http://docs.basho.com/riak/2.0.0pre20/dev/advanced/bucket-types/ for discussion.

Borrowing the to-do list from https://github.com/basho/riak-ruby-client/issues/112

Bucket type CRUD

• [ ] Create bucket type properties
• [ ] Read bucket type properties
• [ ] Update bucket type properties

Object/Key CRUD API

• [ ] Create objects in a specific bucket type
• [ ] Read objects in a specific bucket type
• [ ] Update objects in a specific bucket type
• [ ] Delete objects in a specific bucket type

Secondary Indexes

• [ ] Query secondary indexes in a specific bucket type

[dmitrizagidulin]

See also https://github.com/basho/riak-python-client/pull/315 (Python client bucket type implementation)