Feature Extents

Derive feature extents. Currently stashing in ```cleaned_trails``` as a space delimited string, e.g.: 

```extent: "-149.575773199 61.3912400675 -149.56863907 61.3943977306"```

Sample data:

![image](https://cloud.githubusercontent.com/assets/274668/4692062/e3d0bcae-5748-11e4-901b-061a25843707.png)