This fixes issue #244.

This requires a parameter to be accepted by the block passed to
Geocoder.configure, which means you don't need to remember that the
method is called "config"--you can call it whatever you want.
This behavior seems more standard for configuration blocks.

Geocoder.coordinates sometimes returns nil which causes crashes in the distance
and bearing calculations. By returning a point array of [ NaN, NaN ] this
distances will be also NaN which seems to be a sensible result. If the argument
to extract_coordinates has an unexpected form, also return [ NaN, NaN ].

It's not an ideal name either, but it's short and it's slightly more
specific than "method".