Canvas.update allows one arg. FPS is displayed nicer.

Because Canvas.update did not have any args, it could
not be used as an event handler. Now it can, which makes
attaching e.g. a timer to it much easier/cleaner.
Remove canvas.update from on_draw on all examples.

Use timer instead.
flake
add docs to canvas.update
Make examples use timer more effectively

Made some examples use 60 Hz
Make examples use Timer('auto', ...)
Better canvas.measure_fps
flake
Merge pull request #454 from almarklein/examples-60-fps

Let all dynamic examples run at 60 fps
FIX: Nest freetype imports
Misc small tweaks and fixes.

* Updated readme to stick to 70 chars linewidth. Also minor
  changes to allow copy-pasting it into the index.rst for website.
* Make imshow not get stuck on building gallery
* Gallery building use Canvas context manager.
flake
Merge pull request #465 from almarklein/misc-tweaks

Misc small tweaks and fixes.
Merge pull request #466 from almarklein/misc-tweaks

flake remaining from #465
Delay program validation until Texture units are asigned.

Hopefully this fixes #459. Also see #458.
Merge pull request #467 from almarklein/delay-program-validation

Delay program validation until Texture units are asigned.
Changed README from markdown to rest.
Merge pull request #469 from rossant/readme-rest

Changed README from Markdown to ReST
Merge pull request #463 from Eric89GXL/fix-ft

FIX: Nest freetype imports