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
Merge branch 'master' into new-visuals

Conflicts:
	examples/demo/gloo/imshow.py
	examples/demo/gloo/imshow_cuts.py