Abstract: A generally u-shaped first structure comprises a first lateral member and two first legs. A generally u-shaped second structure comprises a second lateral member and two second legs. Two connecting legs are provided, each connecting leg being pivotally attached to one of the first legs and to one of the second legs, such that the first structure remains generally parallel to the second structure as the first structure is pivoted toward and away from the second structure. A generally u-shaped third structure comprises a third lateral member and two third legs. The two third legs of the third structure are pivotally attached to one of the second legs of the second structure. A pair of support members extend between the second legs of the second structure. Each of the support members has a plurality of spaced pins extending there-from. A pair of arms is provided, each of the arms having a first end pivotally attached to one of the third legs of the third structure.