Added initial cut of basic scripting support, introducing an osg::Script object to wrap up the individual scripts, osg::ScriptCallback for assigning a Script as node callback and an osg::ScriptEngine base class that plugins implement to provided support for specific scripting languages.

Provided are lua, python and V8 (for javascript) plugins that just open up enough of a link to the respective libs to run a script, there is no scene graph <-> script communication in current implementation.