Response to GH issues #579 and #663.

The result of the lookup in maxmind_local is stored at test/test_helper.rb to avoid the inclusion of maxmind's database in repository.

Make Geocoder.configure accept a hash (now preferred) and add
Geocoder.config method (for reading config). Deprecate old syntax.

This requires changing the way stubs work, so that they only simulate
actual HTTP calls, not cache hits.

Allow lookups to define a cache key (defaults to the query URL) and
define Yahoo's as the non-OAuth-encoded request URL.

Also move some lookup-related methods to Geocoder::Lookup module.

Removed set_defaults from all tests and replaced with teardown to reset the defaults.

Signed-off-by: James McCarthy <james2mccarthy@gmail.com>