Compute absolute paths and use this to load files that aren't namespaced well

Using this gem with school_days causes a problem because of their similar folder structure: they both have:

extensions/
  date.rb
   fixnum.rb

require 'extensions/date.rb' was NOT loading business_time's date.rb (I can only guess it was loading school_days date.rb again).

This commit computes the absolute paths (via File.dirname(__FILE__)) to avoid these kinds of load errors