pgraster input plugin

A new input plugin for postgis raster, see #1660
In its current state the plugin only supports 8BPP rasters of either 1 (grayscale) or 3 (RGB) bands.
No automated tests are included yet, but manual test has been performed with default imports
of datasets from http://www.naturalearthdata.com

No attempt is made at supporting "nodata" values.