Adding download kml service

This PR adds a service that will take a filename and a kml string send to it and create a file on the system. The URL to get this file is send as a response. This can then be used to fetch the created file. This is needed for better kml export support in IE9 and Safari.

Note: the files created are deleted after 1 hour (check is done on each request).

A PR in geoadmin will follow that uses this service.
simply
<pre>
kml = spec.get('kml')
</pre>
if not defined, kml will be equal to None