support downloading to different formats

[gubuntu]

Support for downloading to different formats from GeoNode. Similar to above, we will use OGR2OGR on the server side to rewrite a chosen layer into the user’s preferred format (e.g. KMZ) before it gets downloaded to the user’s computer.

see also inasafe/inasafe#1243

[gubuntu]

I propose bringing this into Sprint 1 @lucernae since it has become necessary on http://geonode.inasafe.org to be able to download data in usable formats. e.g. A user of that instance will not be able to use a tiled layer (which is currently the only raster 'download' option) in an InaSAFE training session.

cc @Gustry @ismailsunni @timlinux

[Gustry]

which is currently the only raster 'download' option

For a quick fix, we can make an url to the original raster layer (.tiff or .asc)

[gubuntu]

partly addressed by #159 and #181.