Add an error-prone compiler to plexus-compilers, so error-prone can be e...

error-prone is a static analysis library that acts as a javac "plugin" and fails the compilation when obvious coding mistakes are present, see http://error-prone.googlecode.com

This change adds a new plexus-compiler component. It's duplicated from the JavacCompiler class in plexus-compiler-javac, which makes the diff long. It's best to compare against JavacCompiler.
I could instead extend from JavacCompiler, but it's awkward since I need a different value for JAVAC_CLASSNAME among other things. It would also require a bit of refactoring of JavacCompiler to be more extensible.

Note that I haven't added an automated test yet. I'd like to get some feedback if this approach looks good first.