Reorganize visuals layer

Reorganization to make visuals a top-level package.  Fixes #448.
Only one example currently works; I'll deal with the rest after feed back on a couple of issues: 

1. Any complaints about the current structure?
2. Poll: should `transforms` be top-level as well?
3. There are many modules for users to import, so it would be nice to pick one place where most of the common names are imported by default. Currently there is a bit of this going on in `scene.__init__`, but it's a mess. I'd like to make many of the examples look like:
```
import vispy.scene

canvas = vispy.scene.SceneCanvas()
line = vispy.scene.Line(...)
line.transform = vispy.scene.AffineTransform()
```
`vispy.plot` should do something similar, but with a more extensive set of imports aimed toward scientific vis. 
Yes, but it is hard. For one, when are you going to delete them? You can detect (more or less) that a Python object is deleted, but to delete the corresponding GPU object, the context must be active ...

With the lately added context awareness and GLIR, it should be easier, so I may have a stab at it someday soonish. But I prefer to wait with this until the current system looks stable (though potentially memory leaky).