Closes #31: queue pool limit of size 5 overflow 10 reached

Squashed commit of the following:

commit c370d21facae2d1741ae02ef95f83e2d8618709a
Author: Olivier Yiptong <olivier@olivieryiptong.com>
Date:   Tue Nov 4 11:38:04 2014 -0500

    tidy up test_ingest.py by moving image tile declaration to fixtures

commit 57537929cd39382593521317ac350967d21daff5
Author: Olivier Yiptong <olivier@olivieryiptong.com>
Date:   Tue Nov 4 11:31:24 2014 -0500

    explicit transaction rollback in the case of insert failure in ingest.py

commit 04d1c12f5c38e2e38e44d8151d64f69ff7c8e851
Author: Olivier Yiptong <olivier@olivieryiptong.com>
Date:   Tue Nov 4 11:30:28 2014 -0500

    remove print statement in test_ingest

commit 205a5a33008bdc0a8ce99386b99a5b5f4b255b81
Author: Olivier Yiptong <olivier@olivieryiptong.com>
Date:   Fri Oct 31 10:28:33 2014 -0400

    use only one connection for all tile ingestions

commit c7028936e02d29dcbe8fa2a8356ae625aceee896
Author: Olivier Yiptong <olivier@olivieryiptong.com>
Date:   Thu Oct 30 22:29:20 2014 -0400

    PEP8 improvements

commit f9c04604d15c1d7722d83e8a151d4cf0fc4777ec
Author: Olivier Yiptong <olivier@olivieryiptong.com>
Date:   Thu Oct 30 22:21:48 2014 -0400

    close connections when done with a tile in ingest.ingest_tiles