A few improvements

Hi @willmorgan, here are a few improvements I've made.

Sorry they are all bunched up - I consider them fairly minor, and they are all backwards compatible.

Happy to split up into separate pull requests, if needed.
It is (see https://github.com/BetterBrief/silverstripe-googlemapfield/pull/14/files#diff-ba6222cc563929b288a350c62890347aR18). I think it has to be done at the last minute to that a dev can change the setting whenever they like and not just on construct.