NOJIRA - Define Notifications constants into API

This is usefull to use Notifications into other portlets
which can provide Notification events.
Warning this commit wasn't tested but should be enougth to provide the
elements needed into another portlet after importing
notification-portlet-api dependency.
Merge pull request #5 from GIP-RECIA/NOJIRA-DefineCstIntoApi

NOJIRA - Define Notifications constants into API