Add UK (well, GB) postcode geocoder

[artfulrobot]

This adds support for a UK postcode geocoder. It does not add the data though as that's a 103MB sql file which exceeds GitHub's size allowance. The README has instructions for getting the data.

Of note:

• works on a db table
• allows missing/wrong spaces in postcodes, and corrects them
• I implemented a new provider since getting the existing db table one to play fair would have meant potentially breaking existing installs (assumptions over column names, numerical format of postal_code etc.)
• existing tests pass; new tests added for UK data.

[artfulrobot]

(by the way, I can get the full GB data sql down to 14MB using xz. Is it worth adding that to the repo, or best to let people grab their own from source?)

[eileenmcnaughton]

@artfulrobot re packaging the data - I guess if you do add it then it will still require steps that someone doing a UI install would have to do outside the UI - ie it will require what we would think of as sysadmin steps either way so probably fine as is

[eileenmcnaughton]

I think the charset needs changing & this is otherwise mergeable

[eileenmcnaughton]

/test

[artfulrobot]

Thanks for the review @eileenmcnaughton