openshift-java-client: get rid of external class to (de)code base64

We're currently using a class that we copied from a different project to (de)code base64. This contradicts the fedora project which wants to package us into an rpm. They suggested to package the full library instead.
Since we dont want to agument the numbers of dependencies, I'll switch to JDK means to (de)code base64. This will bind us to >= JDK6 which is IMHO perfectly fine.