FEAT: Generalized n-dimensional array padding

See merged [NumPy PR #3329](https://github.com/numpy/numpy/pull/3329) for full discussion. This is a long one, but it had to be.

This PR exposes NumPy 1.8+ padding functionality to all users of `scikit-image`, regardless of their personal NumPy install version. The improved version (faster in general but **much** better scaling with dimensionality) written by yours truly, to be introduced in NumPy 1.8, is used. It was a lot of fun to write!

After this gets accepted, I'll go through and convert the various functions which use inconsistent, private padding functions over to the public one introduced here.