Add support for custom compiler class names

Created by: kageiit

Currently, the jar backed javac hardcodes com.sun.tools.javac.api.JavacTool as the implementation of JavaCompiler

https://github.com/facebook/buck/blob/8e3c38fcf5403808aa0529afe16b8e9978c9b427/src/com/facebook/buck/jvm/java/AbstractJavacOptions.java#L149

Proposal is to open that up with a compiler_class_name option that can work with javac_jar option.

This lets us integrate alternative compilers like Google's Error prone (https://github.com/google/error-prone) into the build process