Improve test and rdoc process

Handle minitest version 4 and 5, and handle absence of rdoc.
Add EXIFR.logger to log exceptions
Make Time adapter interchangeable

This change allows for the use of Time.zone.local(..) for applications
using ActiveSupport for time zones;

  EXIFR::TIFF.mktime_proc = proc{|*args| Time.zone.local(*args)}
Add Orientation#to_sym

As requested in GH#33.
Release 1.2.0