Remove unused `require 'time'`.
Use Ruby's ISO8601 time formatter and parse times.

This could be considered a breaking change, as it modifies the string generated
for a Time, to include a colon separator between the hour and minute of the
time zone, so +01:00 instead of +0100.
Merge pull request #41 from tomtaylor/iso8601-times

Use Ruby's ISO8601 time formatter and parser