- define checkstyle configs in as few places as possible
- use checkstyle:check since it has defaults that match what we want (fail after logging to console)
- modules that have current failures set property checkstyle.failOnViolation to false.

[core dynamodb googledatastore infinispan] Fixing pmd:UnnecessaryConstructor - Unnecessary constructor, squid:EmptyStatementUsageCheck - Empty statements should be removed

Addressing the last CR comment.

[googledatastore] Add cause to DBException

Copyright statement, better exception messages, etc.

1. Move documentation from the properties file to the README.md
2. Addressed a couple of comments in the code.
3. Added the checkstyle plugin and addressed all the violation (except
the TODO comment violation. I'd like to keep a TODO item in the code).

[googledatastore] Output debug logs in one line instead of multiple lines.

Will implement scan separately in the next pull request.

Add support to allow users to group entities in two different ways, and add documentations for the overall googledatastore.properties file.

Add a DB client for Google Cloud Datastore. This initial commit includes implementation of all CRUD operations (modulo scan), and integration into the rest of YCSB (pom.xml changes, etc)

Next few commits will include Scan/query support, single entity group support, and better documentation.