fixed attachment handling, added "replication damping"

Hello Nick!

I finally managed to start working with GIT and create this pull request.

As mentioned in my "issues", this code 

- fixes some problems of attachment handling in express-pouchdb and
- contains a (hopefully temporary) workaround for the broken conflict management and replication handling of PouchDB

Please, let me know if everything looks ok (which includes the GIT operations required to contribute to your wonderful project!)

Thanks in advance for your effort!

Kind regards,

Andreas Rozek