Remove .env.sample file from gem to reduce confusion.

This should be set in the application e.g. zoningcheck-rails. For
automated testing this will be handled internally anyway.
Greater flexibility with protocol, site url; read constants from config.

Constants like default locale and token path should be in the config
file in case we need them elsewhere and so all the config setting is in
one place.

Other constants are variable depending on whether we're in development,
test or production so should be read from ENV.