Make RiakObject (and all it's stuff) to be Serializable

[srgg]

Description

The RiakObject as well as all it's attributes were marked as Serializable and serialVersionUIDs were added accordingly.

Related Issue

Motivation and Context

In Spark Connector project there is a need to use RiakObject as a default mapped type for the Riak KV RDD.

How Has This Been Tested?

Additional test case has been added in the scope of this PR.

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [x] My code follows the code style of this project.
• [x] I have read the CONTRIBUTING document.
• [x] I have added tests to cover my changes.
• [ ] All new and existing tests passed.
• [ ] Javadoc comments for any new public classes, interfaces, etc.
• [ ] A basho_docs PR for new or changed features (basho/basho_docs#???)

Pull requests that are small and limited in scope are most welcome.