(#667) - slash in doc id

This is proposed solution for slash in document id. This PR includes minor API change:

`db.getAttachment(id, [callback])` -> ` db.getAttachment(docId, attachmentId, [callback])`

This needs thorough review and probably squash before getting merged.

Also: the part commented out in pouch.utils.js could be (I greped and tested; safely?) removed.