Squashed 'lib/Yoozi/Notification/' changes from 6fd83d6..0974617

0974617 Abstract Message.

git-subtree-dir: lib/Yoozi/Notification
git-subtree-split: 09746175a5ab40db181cc6106c7f15ede5a2d216
Merge commit '1578646d57ba7b3672d15d0cb5c66254e4517a66'
Content should always return value.
Set Closure to be optional.
Add test cases.
Bump version 1.0.4: initial release for module notification.