Basic xkcd support for mpl.

First, this PR depends on #482, so the last two commits are relevant for this feature.

Essentially I am aplying xkcd style to Lines and Text... probably we can add more things later, but for now, I think is enough to be pushed. The hand-written function was taken from Jake post (with some custom parameters).

To use it, you just to pass a new argument to the `show_bokeh` function: `xkcd=True`.
I also added an example (xkcd over a ggplot example) and I pasted the resulting figure here for reference.

![xkcd](https://cloud.githubusercontent.com/assets/1640669/2627143/657dcb30-bdfb-11e3-9639-48d7683eb34d.png)