Deprecate the RssHelper.
Deprecate the RssHelper.

This helper is infrequently used, and is better suited to being a view.
We don't have a replacement view class yet, but we can ensure that
RssHelper doesn't come along in 4.x by deprecating it now.

Refs #2108

garethellis36/cakephp,ADmad/cakephp,dakota/cakephp,ndm2/cakephp,cakephp/cakephp,dakota/cakephp,ndm2/cakephp,davidyell/cakephp,mvdriel/cakephp,ADmad/cakephp,mvdriel/cakephp,tenkoma/cakephp,CakeDC/cakephp,tenkoma/cakephp,ndm2/cakephp,petteyg/cakephp,ADmad/cakephp,CakeDC/cakephp,CakeDC/cakephp,petteyg/cakephp,garethellis36/cakephp,ADmad/cakephp,cakephp/cakephp,mvdriel/cakephp,garethellis36/cakephp,davidyell/cakephp,dakota/cakephp,petteyg/cakephp,tenkoma/cakephp,davidyell/cakephp,CakeDC/cakephp,petteyg/cakephp,dakota/cakephp,ndm2/cakephp,tenkoma/cakephp,cakephp/cakephp,mvdriel/cakephp,cakephp/cakephp,garethellis36/cakephp