Add ability to specify indexing in CounterData Entity class

[sappenin]

Currently, if a library user wants to index (e.g.) the CounterData.CounterStatus field, it's not easily possible. We should introduce a new mechanism to allow people to specify which fields of CounterData should be indexed in the Datastore and which fields should not.

Plausibly, this could be accomplished with a combination of Objectify's If.java interface (essentially a poor-man's interceptor framework) and objectify-utils' annotation-based config see here for the Money.java annotation.

[sappenin]

This is complete and will be available in the 1.1.1 version. The default indexing option is to not index any of these fields, so compatibility with 1.1.x is acheived. Hence, adding to version 1.1.1.