Exposure export tool

This branch adds the fully functional exposure export tool, which addresses https://bugs.launchpad.net/oq-platform/+bug/1182426.

https://bugs.launchpad.net/oq-platform/+bug/1185813 is also fixed by this pull request; see https://github.com/gem/oq-platform/commit/0099d4a36f4913c725cd16e778e5cc1f968c7e15.