Remove native scripts and replace with script engine example

[rjernst]

Native scripts are heavyweight things that require creating a plugin. They are used by advanced users who need access to special code (eg a custom library to do something not available in the exposed methods/classes in painless or groovy). Internally, we have a single NativeScriptEngine which handles taking the registered native scripts, and making them accessible. But writing a NativeScriptFactory isn't much different than writing a ScriptEngine.

I think we should simplify scripting internals by removing NativeScriptEngine, and documenting instead how to create a ScriptEngine for this advanced case.