Create a custom ScriptContext

The current ScriptContext in use with the ScriptEngine is the SimpleScriptContext. It doesnt' allow for additional scopes which might be useful when dealing with the ScriptEnginePluginAcceptor which allows plugins to inject "global" bindings into the a ScriptEngine instance (e.g. the "hdfs" object with the hadoop plugin).

We currently inject this into the GLOBAL_SCOPE of the ScriptEngine which at the moment doesn't seem to conflict with anything, but I wonder if more separation is required.

Another alternative would be to manage these types of "globals" with new methods on the DependencyManager interface.