Download calculation results as files

Here is the trick:

- /v1/calc/result/ID visualizes the calculation result on the browser
- /v1/calc/result/ID?export_type=xml download the result as a file

If the given result cannot be exported in the given export_type (because the corresponding importer
has not been implemented yet) you get a 404 and a clear error message.

For the tests, see https://bugs.launchpad.net/oq-engine/+bug/1393733