Visual verification tool: ascii map

To be merged after #20.

This is a silly little tool which can be used to visually verify that the dataset which has been downloaded is sane and being parsed correctly. When run, it generates a little ASCII-art elevation map. If the output looks like the world map then you can have some confidence that the dataset has downloaded correctly.

Sample screenshot:

![ruaumoko-elevation](https://cloud.githubusercontent.com/assets/146860/4751176/d5f03c90-5a9a-11e4-9f31-24e527a6486c.png)