Added a new field to task status, changes_pending

This will provide a rough idea as to how work is remaining in a replication.
It's computed by subtracting the current committed seq from the current update seq.

BugzID: 16058
Merge pull request #15 from cloudant/16058-pending-doc-count-active-tasks

Added a new field to task status, changes_pending

BugzID: 16058