Add missing WrappedSocket.fileno method in PyOpenSSL (issue #188)

This fixes (at least for myself) issue #188 where a missing fileno() method in the WrappedSocket object is required later in urllib3.util method is_connection_dropped() (or more precisely by the poll object select.poll.register() method called there).

I could try to provide a test though I'm not sure where to start, here for preliminary review if you think this fix is appropriate.

From Python documentation (http://docs.python.org/2/library/select) for poll.register method:

> ...
>  fd can be either an integer, or an object with a fileno() method that returns an integer.
> ...