GEOS-6514 Restrict MIME types for WMS GetMap and GetFeatureInfo requests

The JIRA issue is here

http://jira.codehaus.org/browse/GEOS-6514

The pull request contains

- core code 
- test code
- admin GUI (WMS Service page)
- documentation



The copyright here should say just "2014", not "2001 - 2013". The proper form is "year created - year last modified", in this case they are the same.
This should be adjusted in every new copyright header (if there are others, I won't repeat this comment)