Return a signal of signals for both -rac_getData and -rac_getDataStream

Both -rac_getData and -rac_getDataStream now return a signal of signals. The
first is a signal that captures the fetch's percentage progress. The second
signal sends the NSData or NSInputStream when ready. For callers that just want
data, they can use:

    [[[file rac_getDataStream] skip:1] flatten]