Add caching and app identification to adhere to usage policy

[artfulrobot]

Nominatim's usage policy says two things that we weren't doing:

1. Identify the client via User-Agent.
   This PR uses the configured API key if it exists, otherwise it generates a unique hash to identify each unique CiviCRM site.

2. Use caching.
   This PR uses a simple SQL cache.

Notes

I came to this as lookups were failing, and I think it could have been due to the policy (1).

The caching (2) is permanent until the cache is cleared ("Cleanup caches" or such), which is probably enough.

[bjendres]

That looks great, thanks @artfulrobot!! I would release this with a new version, since 1.3 is already in beta, is that ok with you?

[artfulrobot]

@bjendres done. Thanks.

[bjendres]

@artfulrobot Here's a 1.4-alpha1 with your code