Merge pull request #3 from blink1073/canvastools

ENH: Adjusted save_to_file to create a proper composite image
ENH: Cast arrays for numpy compatibility.

The casting rules in newer versions of numpy are stricter, so ensure that we have a compatible data type.
ENH: Improve usability of watershed_demo.

* Replace 0 label with 'Erase'
* Use OK button instead of enter key to calculate watershed.
Fix alpha handling for image saving.
Add save buttons to viewer example.