Add support for setting javac version flags through maven-enunciate-plugin configuration options.

I noticed that enunciate hard-codes a value of "1.5" in the invokeJavac methods. This can break projects that use Enunciate, if they include features that aren't present in Java 5.

I addressed this by adding some hooks in maven-enunciate-slim-plugin that will get "javacSourceVersion" and "javacTargetVersion" from the plugin configuration in a project's pom file. Those variables will be used by Enunciate when invokeJavac is called; if they aren't present, they default to "1.5", which should preserve backwards compatibility and allow the enunciate project itself to compile properly. (The default value is defined in a constant, so it can be easily changed in the future, as well.)