Fix compile error for parser test
Use the offset against GMT for the provided date, not current date

A date in the past may have been under a different absolute offset than we currently are in.
Starting work on a proper unit test suite.
Implemented the test of parsing GMT dates.
Include stdbool and tgmath in the test project's prefix header.
Added unparsing tests, two of which fail because of time-zone mishandling.
Test parsing a third variant of this GMT date.
Merge pull request #10 from zacwest/fix-daylight-savings-time

Fix daylight savings time unparsing (at least for Pacific Time). Refs #6.
Merge branch 'master' of https://github.com/boredzo/iso-8601-date-formatter