Streams-204 | GooglePlus Comments Processor

Added in a Google Plus Deserializer and Processor for Comments. The processor takes any comments found for the passed in Activity (via a to-be-seen provider) and appends them as ActivityObjects. A total "comment_count" attribute is also stored in the "extensions" section of the Activity.

This PR builds off of: https://github.com/apache/incubator-streams/pull/114 and https://github.com/apache/incubator-streams/pull/115