Choose scalehint computation for WMS 1.1.1

Used @mpazos pull request:
https://github.com/geoserver/geoserver/pull/244
and see:
http://jira.codehaus.org/browse/GEOS-5830

As discussed on the mailinglist:
The 'admin' can choose to have the scalehint values as 'denominator' values or as 'units per diagonal pixel' (as  advised in the wms 1.1.1 spec). 
Default is 'denominator' because it's the best of both.
Option is added to the WMSAdminPage

Only needed for WMS 1.1.1 capabilities doc, later version's use min/max scaleDenominators.