Base on new build plugin api

Uses the brand new javassist-build-plugin-api.
Also upgrades version number to 0.3.0
I based that on this 

https://github.com/gradle/gradle/blob/ccddc438ce09293d84030ebe31668d739c8a228a/subprojects/language-jvm/src/main/groovy/org/gradle/api/tasks/compile/AbstractCompile.java#L39