SVG interpreter

Add svg interpreter :)

by using the magic `%svg` with your svg code, zeppelin can now drow it :)
example:

![screen shot 2014-11-04 at 7 38 41 pm](https://cloud.githubusercontent.com/assets/3139557/4898267/df74e658-640e-11e4-98fb-9a0b458fa32e.png)

![screen shot 2014-11-04 at 7 39 07 pm](https://cloud.githubusercontent.com/assets/3139557/4898268/e36f9bcc-640e-11e4-89ca-74ea378527cb.png)