Codegen annotation processor

The codegen as an annotation processor that can be executed with the java compiler removing the need to wrap it with a Maven plugin. It can able to make the javac build fail and report the faulty element as javac would do, for instance:

    [INFO] --- maven-compiler-plugin:3.1:compile (default-compile) @ vertx-core ---
    [INFO] Changes detected - recompiling the module!
    [INFO] Compiling 240 source files to /Users/julien/java/vert.x/vertx-core/target/classes
    [INFO] -------------------------------------------------------------
    [ERROR] COMPILATION ERROR : 
    [INFO] -------------------------------------------------------------
    [ERROR] /Users/julien/java/vert.x/vertx-core/src/main/java/io/vertx/core/Vertx.java:[120,8] void method can't be marked with @CacheReturn
    [INFO] 1 error
    [INFO] -------------------------------------------------------------
    [INFO] ------------------------------------------------------------------------
    [INFO] BUILD FAILURE
    [INFO] ------------------------------------------------------------------------
    [INFO] Total time: 1.660s
    [INFO] Finished at: Fri Jul 11 00:43:24 CEST 2014
    [INFO] Final Memory: 15M/325M

It also decouples codegen from vert.x master (it forks a few vertx classes as stub for testing purposes).