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. 
should probably go through the `setter` (`self.dpi = dpi`), and put `float(dpi)` in the setter -- it's an easy way to ensure that the user / `get_dpi` has passed something sensible