Added osgDB::ReaderWriter::Options class to the ReaderWriter base class, support

for settings options in osgDB::Registry, and added the paramter to all of the
reaader/writer plugins.  The Options structure by default has an string attached
for packing basic options, however, it also can be subclassed to encapsulate
any users defined option data. In the later case both the client code *and*
the plugin need to be aware of subclass, the plugin will need to use
dynamic_cast<> to assertain its type.

src/osgPlugins/bmp/ReaderWriterBMP.cpp

@@ -290,7 +290,7 @@ class ReaderWriterBMP : public osgDB::ReaderWriter
         virtual const char* className() { return "BMP Image Reader"; }
         virtual bool acceptsExtension(const std::string& extension) { return extension=="bmp"; }
 
-        virtual osg::Image* readImage(const std::string& fileName)
+        virtual osg::Image* readImage(const std::string& fileName, const osgDB::ReaderWriter::Options*)
         {
 
             unsigned char *imageData = NULL;