Add calculator plugin!

One of my favorite Alfred features is being able to type in simple expressions and copy the result to the clipboard. Here's my attempt to do that!

try typing `calc 4+5` it'll show you:

![screenshot from 2014-12-03 15 03 51](https://cloud.githubusercontent.com/assets/326829/5290664/cf01b656-7afd-11e4-9d28-6253201176a4.png)

Or calculate something fancy:

![screenshot from 2014-12-03 15 04 25](https://cloud.githubusercontent.com/assets/326829/5290669/d7575c20-7afd-11e4-8ab0-9a6020a438f3.png)

Very complex equations are possible thanks to sympy, the only requirement. This script even let's you know if you need to install it.