TWSignedRequest.m - provide a timeout to prevent an app using this from spinning forever 

got into a situation where the twitter auth signed request was not being
returned, probably due to temporary glitch on the twitter servers.  it
would do the first part but not this second part.  rather than hanging
forever, an 8 second timeout will at least allow a proper NS Connection
error to be reported up the chain, and a proper app error handler will
report the timeout … and the user can just try again later.