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.
I assume you're performing a POST request, you should proably add
<pre>
request_method='POST'
</pre>
in view_config