Add WmsStore and WmsLayer as noted here http://geoserver.org/display/GEOS/GSIP+47+-+WMS+cascading 

Link:http://geoserver.org/display/GEOS/GSIP+47+-+WMS+cascading 

Useful when cascading external WMS layers in GeoServer by adding them 
through "Add new Store"->"WMS - Cascades a remote Web Map Service"

Creating the layers through REST is not supported by GeoServer yet as
noted here http://projects.opengeo.org/suite/wiki/upload so layers must 
be manually published from inside GeoServer

This is my first pull request ever, so be gentle please!