Add postcode anywhere uk provider

This is a fairly simple addition to **Geocoder**, it adds a new lookup for [Postcode Anywhere UK Geocder](http://www.postcodeanywhere.co.uk/Support/WebService/Geocoding/UK/Geocode/2/) service.

This Postcode Anywhere API implements a standard definition of a Geocoding service; "given a location, return where it is in the world". 

For example a query for "alton towers", the service returns the following response:
```json
{"Items":[{"Location":"Alton Towers, Staffordshire","Easting":407060,"Northing":343607,"Latitude":52.9897,"Longitude":-1.8963,"OsGrid":"SK 07060 43607","Accuracy":"Standard"}]}
```

With the only caveat that this uses the **UK only** service, this is because more data is returned than the international service available.

This relates to the respective changes to hsa-platform in this PR https://github.com/homeserve-alliance/hsa-platform/tree/postcode_anywhere_geocoder