Add forgotten import in the Manifesto example

Signed-off-by: Balthazar Rouberol <balthazar.rouberol@mapado.com>
Use FieldsPreparer instead of Preparer in Manifesto example

Signed-off-by: Balthazar Rouberol <balthazar.rouberol@mapado.com>
Merge pull request #15 from brouberol/master

Add forgotten import in the Manifesto example