Safari extension

Here's a Safari extension for Pesticide. I've already submitted it using the URLs from my fork, but if you merge this in I'll update the extension to point URLs to your repo instead.

If you'd like to try it now, you should be able to go the the raw version of the `.safariextz` file and that will download it to your computer.

Edit: Click this link [https://github.com/paulmolluzzo/pesticide/raw/safari_extension/pesticide_for_safari.safariextz](https://github.com/paulmolluzzo/pesticide/raw/safari_extension/pesticide_for_safari.safariextz) in Safari to download the extension, when you open it it'll ask if you want to install it.