[GEOS-6376] Moved wms quantizer classes from geoserver to geotools and the pngj encoder to imageio-ext (Backport on the 2.5 branch)

Backport to the 2.5.x branch.

Note that this pull request requires that the pull request https://github.com/GeoWebCache/geowebcache/pull/236 on GeoWebCache is merged before.

Please look at the JIRA: https://jira.codehaus.org/browse/GEOS-6376