Add view changes functionality

The changes in this repo are primarily for reading changes feeds. It adds functionality for couch_changes:handle_changes to handle both view and db changes queries.
Probably not in this case as changes enumeration is a short lived thing. For continuous changes feeds we re-enter this function periodically. Leaving off the ?MODULE means we wouldn't have to handle upgrades between every invocation of changes_enumerator/2. Granted it appears to be an accidental change here so that could just be an oddity.