Adding a kwarg to `set` for custom gridweights

Here's a small PR for allowing the user to specify a custom grid line width. Current values are maintained as a default.

How should we go about the tests? my current setup is pretty noisy when it comes to the notebooks. Here's an nbviewer link (`In[]` cells 7 though 9):
http://nbviewer.ipython.org/gist/phobson/8231880