FEAT: Generalized n-dimensional array padding
This PR exposes NumPy 1.8+ padding functionality to all
users of scikit-image, regardless of their personal NumPy
version.

The improved (much better scaling with dimensionality)
version introduced in NumPy 1.8 is used.
Merge pull request #577 from JDWarner/add_pad

ENH: Generalized N-dimensional array padding.