Canvastools

Major overhaul of tools for interacting with the scikit-image viewer.

* Add skimage.viewer.canvastools subpackage with line and rectangle selection tools
* Refactor line profile plugin to use line tool
* Fix viewer canvas
* Fix blitting behavior
* New plugins (ColorHistogram, Measure, Crop)

To get a feel for the changes, see new examples in `viewer_examples/plugins` located in the root directory of package.