Add gaf_viewer to dev env

Addresses https://bugs.launchpad.net/oq-platform/+bug/1224354

https://github.com/gem/oq-platform/pull/72, https://github.com/gem/oq-platform/pull/73, https://github.com/gem/oq-platform/pull/75, and https://github.com/gem/oq-platform/pull/78 should merge first.