Implement Geocoding by Postal Codes for Germany

[Detsieber]

This solution is similar to the implementation of OK postcodes.

[Detsieber]

@eileenmcnaughton : This commit is active for more than a year in a heavily used production environment (however, with Geocoder 1.7), and works well.

However, there are still a few issues:

1. It ships together with the complete dataset of German postcodes. AFAIK there are no license issues for that - but if you would prefer to better load the data from an external source, this would still have to be implemented. (I would think it would be better to ship the data with the extension, or at least provide an easy way to load the data via the UI)
2. no unit tests for German postcodes
3. Update of Readme is missing

Please let me know what you need for merging this!

[eileenmcnaughton]

This was replaced by #38