shapefile downloads now use the taxonomic_cateogires option

[ricardogsilva]

This PR addresses several issues:

• It closes #101 by including the created_on field on the order representation returned by the REST API

• It is also connected to #96 since it adds the backend code for retrieving the email field from the request

• It is also connected to #86 whereby it enables shapefile download requests to take into account the taxonomic_categories parameter. A request like the following (I'm using httpie as a client):

  http POST 10.0.1.95:8000/gfdrr_det/api/v1/order/ \
    notification_email='ricardo.garcia.silva@gmail.com' \
    order_items:='[{"layer": "exposure:cpv_buildings_127", "format": "shapefile", "bbox": "-23.598170,14.890766,-23.4133849,14.96382", "taxonomic_categories": "masonry,wood"}]'

  This request results in the server creating a shapefile clipped to the specified bounding box and with only the features whose parsed_taxonomy attributes match the ones in the request

  Note that similar functionality is not yet available for downloads that request geopackage as the format (currently the geopackage code only takes into account the bounding box, not the taxonomies)