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

Related to https://github.com/GoogleCloudPlatform/kubernetes/issues/1128
@crhym3 Content-type actually seems to be more complicated. "... or if it is an ASCII case-insensitive match for Content-Type and the header field value media type (excluding parameters) is an ASCII case-insensitive match for application/x-www-form-urlencoded, multipart/form-data, or text/plain."

Since the POSTed content-type is usually going to be application/json it looks like Content-Type header will need to be explicitly allowed.