Rename timeinterval/interval to duration.

This is more compliant with ISO 8601.
Merge pull request #59 from benfitzpatrick/timeinterval-renamed-to-duration

Rename timeinterval/interval to duration.
Update for 2014.08.0
Update CHANGES.md
stricter dumper year bounds checking
Merge pull request #60 from benfitzpatrick/dumper.year-bounds-error-adjust

Stricter dumper year bounds checking
Update CHANGES.md
tentative fix
fix tests
add more subtract tests
Merge pull request #61 from benfitzpatrick/iso.fix-subtract-2-greater-than-1

Fix date1 - date2 where date2 is greater than date1