Major tiled raster loading refactor.

This includes deleting GDAL\importing code, this is now handled by data.py,
refactor of FileReaders, rendering tiled rasters in admin site, and
making epsg, xskew and yskew optional in raster layer metadata JSON file.
Remove debugging println statements.
Merge pull request #475 from joshmarcus/test2

 Major catalog and layer loading refactor.