Add option to enable a simple CORS implementation for the api server

Related to https://github.com/GoogleCloudPlatform/kubernetes/issues/1128
I don't like adding this parameter here. This is a specific case of the pattern where you want a request to go through a number of handlers.

For now, if you want to use CORS, you can wrap the handler that this returns in your CORS handler, don't pass a bool, there's no reason to make this function do that for you. (Is there?)

For the future, I can see making some way to install a handler filter thingy in a defaultAPIServer, or setting up a []http.Handler through which requests must pass or something. I think there's a few different libraries that do this that we should look at before writing something.