Support filtering out approved patches.

Sometimes I use qgerrit to list all patches that have a +2 and no -1
votes.  When I do that, I also want to filter out approved patches.
With this change, I can do a query like:

    qgerrit -l russellb -p openstack/nova -a c2,a=0 -w ^russellb

List all changes that are:

  - against nova
  - have at least one +2
  - have not yet been approved
  - have not been reviewed by me
Merge pull request #13 from russellb/filter-approved

Support filtering out approved patches.