SciTools/cartopy — pull request #168 by pelson (2012-12-05 14:36:22+00:00)
Title: Updated code to use the new pre_existing_data_dir config item.

I've added a ```pre_existing_data_dir``` config item which allows one to override the pre-downloaded data directory easily in cartopy. As an example, I had a ```siteconfig.py``` file in cartopy which looked like:

```
def update_config(config):
config['pre_existing_data_dir'] = '/var/tmp_non_exist'
```

I was also able to test the whole thing with:

```
def update_config(config):
import cartopy.io.shapereader
dnld = config['downloaders'][('shapefiles', 'natural_earth')]
print dnld.target_path_template
print dnld.pre_downloaded_path_template

```

This turned out to be quite useful, as I'd forgotten that there was a workaround to the default NaturalEarth target path so that our tests continued to pass. I have also fixed that in this PR (closes #150).