windows: fix tzinfo for timezones without dst

fixes lp:1010050 and closes #2

This patch was taken almost as is from the launchpad merge request by
@ccanepa :
https://code.launchpad.net/~ccanepacc/dateutil/fix-isdst-in-tzwin/+merge/224424

Applyed to the git repo by @jarondl
Windows: Be more lenient about missing timezones, like on posix
tests: switch to nosetest, which allows to skip tests
fix CI on python2.6
CI: continue to fix builds, forgot to import skipUnless on python>2.6
CI: fix unittest import
Merge pull request #7 from dateutil/fix-isdst-in-tzwin

Fix isdst in tzwin
fix timezones without dst in windows, and be more permissive about missing windows timezones.
closes #2