Test coverage

[Stéfan van der Walt]

Hello, everyone

We now have automated test coverage for scikit-image:

https://coveralls.io/r/scikit-image/scikit-image

Most of the newer modules (those that went through rigorous PR
review!) did pretty well, but we are still only at 82%.  If ever you
find yourself with a bit of spare time on your hands, please hunt down
some of these corner cases and write tests for them so that we can
start making our way towards 100%.

Thanks!
Stéfan