Update README

* [x] Added more information about Vispy's current status.
* [x] Added links to developers' home pages.
I wouldn't call this the core of vispy. Also I don't like the distinction of the four subsystems. I'd rather tell what vispy.app provides: a scenegraph with flexible transformation and the ability to write custom Visuals from snippets of GLSL code (or something like that). 

I would also like to hear @lcampagn 's opinion on this.