Refactor use func

This PR basically does three things:

1) `Application.use()` no longer exists. When you create an Application object, you specify the backend. No more backend-less state.
2) There is no more `app.default_app` because it is initially None, which makes things complicated. Instead, use `app.use_app()` to get the default app instance. 
3) There is now a `vispy.use()` function that acts as a proxy for `app.use_app` and `gloo.gl.use_gl`. 

Some explanation on 3: One can now do e.g. `vispy.use('glfw pyopengl')`. A more interesting example is `use('pyqt4 es2')` in which case the app backend must know that it should use EGL (instead of a desktop context). And later on ... `use('ipython_notebook')` or `use('browser'). This PR thus paves the way (a wee bit) towards use of ES2, Angle and browser-based rendering.



What about allowing for `use(app=None, gl=None)`, that defaults to something reasonable? Then if someone wants `ipython`, if they do `use('ipython')`, `use(gl='webgl')`, or `use('ipython', 'webgl')` (or whatever names we settle on) it does the right thing, but if they do `use('ipython', 'desktop')` it throws an error (since that's an incompatible `gl`)? We have to do this checking at some point anyway since a user could do `use('ipython desktop')` even in the current system.