Add convenience method to MTLModel to transform JSON dictionaries

There's probably a quicker, in-built way of doing this...

    NSArray *products = [LMProduct transformJSONArray:response[@"products"]];

Contents of `response`:

	{
	    products: [
	        {
	            title: "Apple iPad",
	            ...
	        },
	        {
	            title: "Apple iPhone",
	            ...
	        }
	    ]
	}