Add Alchemist:Measurement#geospatial for use of approximate conversions to and from (arc length) distance and angles on Earth.
Example: 1.mile.geospatial.to.degrees == 0.014457066992474555.degrees
Updated unit tests and readme accordingly.
Refactor geospatial.
make `require 'alchemist/geospatial'` work without having required alchemist first; updated README