ENH: Added "spine_offset"  function to utils

Simple PR to add a `spinesep` kwarg to `utils.despinse`.

Right now, the input is the outward offset in points -- so that leaves all of the tweaking up to the user if the figure size changes. It might make sense to specify it in axes fraction, but that require a little more logic. Feedback is much appreciated.

Tests to follow later.

![capture](https://f.cloud.github.com/assets/1163939/2035437/2246634a-893a-11e3-8239-c9ea1f1ff1cf.PNG)