Add SHP download

[oliverroick]

Proposed changes in this pull request

• Implements Shapefile Export #323
• Add GDAL/OGR installation via Ansible
• Introduces organization.download.shape.ShapeExporter, which exports project data to shapefile format.
• Introduces organization.download.base.Exporter class for common export functionality. Both organization.download.shape.ShapeExporter and organization.download.xls.XLSExporter extend this class.
• Adds shape export option to forms and templates

When should this PR be merged

This PR can be merged immediately. We should, however, check the following:

• VM provisioning with GDAL/OGR works and is tested. I'd test it on another machine as well, just to be sure.
• The copy on the download form should be checked by a native speaker.
• The copy in the README template should be checked by a native speaker.

Risks

None/low risk.

Follow up actions

• The installation of GDAL/OGR alongside PostGIS 2.1 can be problematic. I added the necessary steps to the Ansible playbook. We should test if GDAL/OGR installs correctly on our AWS instances before we roll out shape file export to production (/cc @amplifi)