Redefine/rename UTF-8 API. Add JSONString methods for UTF-8.

Example code from unit test:

        str = "\"\\u0024\"";
        parser->SetInput(str);
        CPPUNIT_ASSERT(parser->Parse() == true);
        obj = static_cast<JSONString *>(JSONAPI::GetValue(parser));
        CPPUNIT_ASSERT(obj);
        type = obj->GetType();
        CPPUNIT_ASSERT(type == JsonType_String);
        value = obj->Get();
        CPPUNIT_ASSERT(value == "\"\\u0024\"");
        char *ret = obj->GetAsUTF8();
        CPPUNIT_ASSERT(ret);
        value = ret;
        CPPUNIT_ASSERT(value == "\"$\"");
        delete obj;