expose class_loader functionality in pluginlib

[mirzashah]

I thought it might be interesting to expose some of the class_loader functionality like introspection of classes and the ability to create plugins from real class name. This could serve as a way for users to stop using the "magic name" for plugins and use the real nae instead (down the line).

nts: How to get users to use new PLUGINLIB_DECLARE_CLASS_SIMPLE macro instead?

[mirzashah]

This was fairly easy to add in. I figured it required modifying rospack but it was actually in that big xml parsing mess (which I cleaned up a little more)