Spine offsets with despine

Someone piped in on my matplotlib email list question about offset the spines. He included a simple function that allows use to offset the spines at any point (not just immediately after creation). Short version is that moving spines makes a call to `ax.cla`. The function just redefines `cla` as `reset_ticks`, moves the spine, and restores `cla` to its original state. Why that works is a great matplotlib mystery.

Sucks that we went through all that work and ended up with a, IMO, subpar API, but this PR cleans it up well.

Question: Should we keep the old `offset_spines` function or is the seaborn API in a state of sufficiently rapid flux that we can get rid of it (a la "there should be one way to do things").