Support output of KML feed for export of location data

This patch allows exporting the latitude and longitude added to a node via the location module as a KML file for use with Google Earth, Marble, and other compatible software.

Unlike CSV or XML feeds, the KML format requires a specific set of fields to be present: name, description, and coordinates. Field labels are used to indicate which fields are to be used. To use this, add the field "Location: Coordinates" with the label "coordinates", and add the placemark's name (e.g., the "Node: Title" field) with the label "name". Descriptions of the placemark are added using a CDATA region which can include arbitrary HTML, which will be rendered by Google Earth.

I'm including a PNG icon to be used with KML files, which is required for the patch.

The patch depends on a minor patch to the location module: #558144: Output KML-compatible decimal degrees →

Further information on the structure of KML files is available at http://en.wikipedia.org/wiki/Keyhole_Markup_Language