add MIT LICENSE

resolve #3
add waffle.io badge
Merge pull request #7 from waffleio/master

waffle.io Badge
tweak badge position
write changelog
impl getEventInternalUrl
modify: open iid url at popup
modify: open iid url at notification
tweak popup link UI
add screenshots on README

close #6
Merge pull request #10 from sue445/issues/8

Wrong issue|merge request numbers in links #8

close #8
[bugfix] Uncaught TypeError: Cannot read property 'name' of undefined
show iid at popup and notification
Merge pull request #12 from sue445/issues/9_show_iid

show iid at popup and notification : close #9
modify support gitlab version

because https://github.com/sue445/chrome-gitlab-notifier/issues/8#issuecomment-27188849
tweak extensiton description
tweak doc