1st Gradle based release, Java 8, Antlr 4.7. Protobuf 2 support only.

This is the as-is migration from Maven to Gradle build system wherein sub-projects broken into separate repos.
In addition, all dependencies, including test ones, are up to date and Java 8 based/friendly. The code itself is not changed with exception of minor upgrade modifications. All existing tests are passing against Protobuf 2.6.1.