Bucket type arguments are confusing

Trying to use `get` against a CRDT bucket gives `not_found` even if the object exists. I expect this is related to the fact that `get` isn't the correct way to grab a data type object, but would recommend giving some other error.

This is true even if specifying the `bucket_type` explicitly as per docs.basho.com:

````
=> #<Riak::Bucket {ruby}>
irb(main):011:0> set_bucket.get(key)
Riak::ProtobuffsFailedRequest: Expected success from Riak but received not_found. The requested object was not found.
	from /Users/John/github/Basho/riak-ruby-client/lib/riak/client/beefcake_protobuffs_backend.rb:84:in `fetch_object'
	from /Users/John/github/Basho/riak-ruby-client/lib/riak/client.rb:241:in `block in get_object'
	from /Users/John/github/Basho/riak-ruby-client/lib/riak/client.rb:333:in `block in recover_from'
	from /usr/local/Cellar/ruby/1.9.3-p374/lib/ruby/gems/1.9.1/gems/innertube-1.0.2/lib/innertube.rb:127:in `take'
	from /Users/John/github/Basho/riak-ruby-client/lib/riak/client.rb:331:in `recover_from'
	from /Users/John/github/Basho/riak-ruby-client/lib/riak/client.rb:303:in `protobuffs'
	from /Users/John/github/Basho/riak-ruby-client/lib/riak/client.rb:240:in `get_object'
	from /Users/John/github/Basho/riak-ruby-client/lib/riak/bucket.rb:99:in `get'
	from (irb):11
	from /usr/local/bin/irb:12:in `<main>'

irb(main):012:0> set_bucket.get(key, bucket_type: 'set_bucket')
Riak::ProtobuffsFailedRequest: Expected success from Riak but received not_found. The requested object was not found.
....
````