Change calculator sources of the extension

Because iGoogle calculator was closed from Nov 2013, so I change the extension to use
 mathjs for caculator, unit converter and https://rate-exchange.appspot.com/currency for currency converter

![2014-01-30_22-10-19](https://f.cloud.github.com/assets/886778/2041182/464337fe-89c2-11e3-85de-e99b3d18bbff.png)