Make TimezonePeriod properties public.

- Make start_transition and end_transition on TimezonePeriod public (now
  that they return classes that are part of the public interface).
- Add tests for start_transition, end_transition and offset (which was
  made public in an earlier commit).
- Remove unnecessary uses of send to call start_transition and offset.
Convert the CHANGES and README files to Markdown.
Replace old RubyForge links in the readme file.
Add documentation comments for TZInfo and TZInfo::Data modules.

Avoids blank pages being generated by RDoc and the licence being used as
documentation by YARD.