Add City Filtering

Fixes #14

Hey @DanielSantoro,
I've built city filtering and added GMT offsets to the list.

Filtering was something I really wanted as it means I can add heaps more cities to the list which I'll do in another PR.

![test](https://cloud.githubusercontent.com/assets/475255/4113706/e4adf92a-3254-11e4-8688-d70b21c225f9.gif)