Add notification support

Currently only implemented in push.go

Should support both Ubuntu and OSX if terminal-notifier is installed
Add notifications for `force test`
Update README.md
Add config for notifications

Rather than always showing notifications, they will default to off and
then can be enabled by the user using `force notify true`
Add gotifier to .travis.yaml
Merge pull request #105 from ViViDboarder/notifications-rebase

Optional Notifications rebase