Add allowsCellularAccess option.

Add allowsCellularAccess option. This option is only for iOS platform but not OSX. The default value is YES.
Isn't this too late to set the stream property? By this time the socket has already opened and made a connection, which could be cellular. I think this needs to be set when the CFStreams are created, before they're opened.

Also, this only needs to be done if _allowsCellularAccess is NO.