make: report errors in running test
Refactor app.use. Application() selects backend on init.

An application thus cannot exist in a backend-less state. Removed
vispy.app.default_app. One should use vispy.app.use() to get the
default app instance.

This is in general a nicer API (I think) and should make further refactoring
towards combining app.use and gl.use easier.
add Application.get_default_app() much more explicit then 'use'
Add vispy.use function: proxy for app.use and gloo.gl.use

So we can do vispy.use('ipythin_nb') and render in the notebook!
Make a test be able to work without any app backends
add test for vispy.use
flake
tickle Travis
app.use -> app.use_app;  gl.use -> gl.use_gl

Plus addressing a few other comments
Define backends in vispy.util._use and import it in app.backends

In this way we do not have to keep two lists in sync
docs
more docs
more docs
tickle Travis
vispy.use() has two arguments instead of single string.
tickle Travis. AGAIN
try again to see if other Travis builds fail too
Fix bug on py33 for PyOpenGL 3.1.0
Fix docs + vispy.use() does not have to know the app backends now
Merge pull request #283 from almarklein/refactor-use-func

Refactor use func