Add additional GMT abbreviations

Merge pull request tkuchiki#23 from tkuchiki/add-is-dst

Add IsDST

Merge pull request tkuchiki#21 from tkuchiki/v0.2.0

V0.2.0

Merge pull request tkuchiki#16 from sashabaranov/master

Invert Etc/GMT±X signs

Merge pull request tkuchiki#11 from scottleedavis/master

added north america dalight savings zones.

Merge branch 'kkavchakHT-timezone-abbreviation-lookup'

Merge pull request tkuchiki#8 from tkuchiki/fix-GetOffset-DST

fix GetOffset (DST is considered) tkuchiki#7

Merge pull request tkuchiki#5 from tkuchiki/fix-FixedTimezone

Fix fixed timezone