added hooks for Schemas

Hi Francois,

For architectural purposes at Kimono Labs, we wanted to be able to add a post "insertAfter" hook on the schema rather than the model, which kept us from repeating code in an application where schemas were utilized in multiple files. 

What are your thoughts? I temporarily created a mongoose-lifecycle2 package but will definitely take it down if this PR is integrated. Tests and documentation has yet to be made.

Cheers,
Shri