Enable setting of content_type for the riakc_pb driver

This change allows the content type of objects to be specified when inserting or updating values, making it possible to generate objects containing text data that Riak Search and Yokozuna can index for benchmarks.

An example configuration file, `./examples/riakc_pb_text.config`, shows how this can be achieved based on a sample text file has also been provided (`./examples/AroundTheWorldIn80Days.txt`). This creates and updates objects with a random, continuous 1000 byte sequence from the text.