Moving the digest hooks outside of JettMailPlugin

This allows other plugins to extend the digest in more of a fine-grained way.
Converting manifest to Elgg 1.8 format; Enforcing some configuration dependencies in code.
Passing notifications array to low level functions so that from guid is available for generating return path addresses.
Adding notification context to jettmail email hooks
Revert "Adding notification context to jettmail email hooks"

This reverts commit fba8c799ff69df70ece7f0f2601e7a720c359d31.
Merge pull request #2 from mapkyca/passing-notifications

Passing notifications
Merge branch 'master' of git://github.com/jumbojett/jettmail