Initial support for D3.js and vega.js plots in the IP notebook.

This builds up on the prototype that I presented in a [blog post.](http://blog.krastanov.org/2013/05/14/sympy-plots-with-d3-in-ipyhton/)

It works only in IPython notebook. I have added a notebook with examples.

The notebook _must_ be evaluated for the plots to appear (due to the hacking around with `publish_javascript`).

There are rumors that this same `publish_javascript` is getting deprecated and a new `_repr_json_` is being developed. In any case, this will be known in the second half of 2013.

Sometimes it is necessary to evaluate a cell twice to get the plots (again due to the hacks with `publish_javascript`).

It works only for lines (`plot` and `plot_parametric`), Custom aesthetics (like color dependent on coordinates) do not work yet, nor most of the options, but they would be very easy to add.

Hovering over a line makes it bigger so one can see it beneath others if the plot contains a lot of information.

Do you want to merge this as it is, or would you prefer that we wait for IPython to implement the necessary dependencies.

![screenshot from 2013-05-17 22 16 19](https://f.cloud.github.com/assets/705248/520156/e294a872-bf2e-11e2-9a19-e23a67309e1d.png)
![screenshot from 2013-05-17 22 15 41](https://f.cloud.github.com/assets/705248/520157/e2c4c4bc-bf2e-11e2-91f7-787e49eee342.png)