[LIBHBASE]: Integrating with maven build and updating protobuf dependency to 2.5.0.

* Added dependency on AsyncHBase 1.5.0-libhbase-SNAPSHOT.
* Updating Protobuf to 2.5.0 as AsyncHBase 1.5.0 requires it.
* Updating slf4j version to 1.7.5.
* Gradle build is currently enabled only on Linux.