Move canny from filter to feature 

This is a little refactoring to move *canny* from *filter* to *feature* keeping backward compatility (https://github.com/scikit-image/scikit-image/wiki/EuroSciPy-2014-sprint-ideas).
Since we're maintaining backwards compatibility by continuing to import from `skimage.filter` until 0.13, is this actually an API change? 

Or should we just add the deprecation warning when used from the previous location and just make the note in TODO.txt?