[Issue:270] Wrong handling of the ImageIO and ImageIO-Ext plugins (Backport on 1.6.x branch)

Backport on the 1.6.x branch of the pull request https://github.com/GeoWebCache/geowebcache/pull/271 related to the issue https://github.com/GeoWebCache/geowebcache/issues/270.

Note that this pull request also requires to change the GeoServer application context file called geowebcache-wmsservice-context.xml.