Reviewed by Maciej.
        
        - Implemented ref-counting of JSContexts by splitting into two datatypes:
        JSGlobalContext, which you can create/retain/release, and JSContext, which
        you can't.
        
        Internally, you retain a JSGlobalContext/ExecState by retaining its
        interpreter, which, in the case of a global ExecState, owns it.
        
        - Also made ~Interpreter() protected to catch places where Interpreter
        is manually deleted. (Can't make it private because some crazy fool
        decided it would be a good idea to subclass Interpreter in other frameworks.
        I pity da fool.)

        * API/APICast.h: 
        (toJS): Added cast for new JSGlobalContext
        * API/JSStringRef.h: Changed vague "you must" language to more specific
        (but, ultimately, equally vague) "behavior is undefined if you don't" 
        language.
        (KJS::Interpreter::Interpreter): Factored more common initialization into
        init()
        * kjs/interpreter.h:
        (KJS::Interpreter::ref): new
        (KJS::Interpreter::deref): new
        (KJS::Interpreter::refCount): new
        * kjs/testkjs.cpp: 
        (doIt): Ref-count the interpreter.

JavaScriptGlue:

        Reviewed by Maciej.
        
        - Updated JSInterpreter to work with Interpreter ref-counting in JavaScriptCore.

        (JSInterpreter::JSInterpreter::~JSInterpreter): Now protected to catch
        manual delete.

WebCore:

        Reviewed by Maciej.

        - Updated ScriptInterpreter to work with Interpreter ref-counting in 
        JavaScriptCore.

        (KJS::ScriptInterpreter::~ScriptInterpreter): Now protected to catch
        manual delete.



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@15437 268f45cc-cd09-0410-ab3c-d52691b4dbfc