Borrows heavily from @Tillsen's solution found on
StackOverflow here: http://goo.gl/RPXMYB

Used with his permission dicussesd on Github here:
matplotlib#3858

This will allow the ticks of colors to be spaced as desired.
Also simplified the math per the brilliant @joferkington
http://stackoverflow.com/a/20146989/1552748

couldn't actually run the test suite b/c python
iesn't installed as a framework.

Need to allow it in __call__ since colorbar can pass a value for clip.

To make it more similar to the other norms

Also remove misleading comment about returning scalars

Since possibly some earlier versions of numpy returned a scalar,
wrap the value in atleast_1d before indexing.

np.interp handles the case vmin == vcenter, we have to add a special case
to make the last test case pass

Fixes