Added popups to murder map

I've added popups to the map that look like this:

![screenshot from 2014-12-23 13 18 57](https://cloud.githubusercontent.com/assets/44195/5533890/48458cec-8aa6-11e4-90d5-248fb1f8fce6.png)

To change what shows in the popup you can just edit the `murderPopupText` function.

Not sure how well this will work for multiple markers at the same position though.

Issue #21