Clarified that you can pass images of any type into Canny filter.
If low and high thresholds are not specified by user, default to 10 and 20 %
(respectively) of range of image's dtype.
Fixed relative threshold: you consider 10% and 20% of the max for range of
corresponding dtype.
Improved style.
Always follow the style guide :)
Included Tony's edits.
Merge pull request #618 from mkcor/filter_canny_passingtype

Canny filter: You can pass an image of any dtype!