Swap replay subjects for direct signals in AFHTTPClient+RACSupport

With this in place, HTTP request operations can be properly canceled and
disposed of.

See "Share the side effects of a signal by multicasting" and "Avoid using
subjects when possible":

1. https://github.com/ReactiveCocoa/ReactiveCocoa/blob/bad9bdae046b273b66ad477deedc078139fc7873/Documentation/DesignGuidelines.md#share-the-side-effects-of-a-signal-by-multicasting
2. https://github.com/ReactiveCocoa/ReactiveCocoa/blob/518ba738feb601e6d43f7a110926246c7ef3a0a1/Documentation/DesignGuidelines.md#avoid-using-subjects-when-possible
Use RAF- prefix for error key constant
Update header docs reflecting change away from replay subjects
Update header docs to indicate cold signals
Merge pull request #8 from kastiglione/swap-subjects-for-signals

Swap replay subjects for direct signals in AFHTTPClient+RACSupport