Add generalized anisotropic spacing support to random_walker

This PR adds the kwarg `spacing` to `skimage.segmentation.random_walker`. By default this is not used, and it is backwards compatible with `depth` - however, under the hood everything now uses `spacing` and is fully general.

There are also fixes to the test suite to eliminate a lot of assumed truncating division which was causing newer NumPy versions to complain about indexing by floats.

This deprecation of `depth` would ideally be included in 0.9

The name `spacing` might be replaced by `sampling`; see the important discussion in #754 and make your opinion known before 0.9 is released!