Update `fabric.Image.filters.Mask` - based on https://github.com/kangax/fabric.js/pull/667 (@aleph1)

Update `fabric.Image.filters.Mask` - based on https://github.com/kangax/fabric.js/pull/667 (@aleph1)
[BACK_INCOMPAT] `fabric.Image._initFilters` is now async, add parameter callback
Add `fabric.util.resolveNamespace` - used to get correct klass in `fabric.util.enlivenObjects`
Doc additions