moved readwrite properties inside an NSStream extension

Apparently the latest version of clang that ships with xcode 6 beta doesn't authorize to rewrite properties in categories if it's not a category from the origin class.

Leading to this kind of warnings: `auto property synthesis will not synthesize because it is readwrite but it will be synthesized readonly via another property`.

I've moved the culprit part inside an extension of `NSStream` to solve those compiling issues.