[Twisted-Python] Deferred documentation rewrite

[Edward Z. Yang]

You can view an initial draft of the rewrite here:

http://ezyang.com/twisted/defer2.html

For reference, this is the planned outline (X means done, ? means
almost done):

X Synchronous to Asynchronous: The Method to the Madness
    X Convert synchronous code to asynchronous code
    X Why asynchronous?
- Deferred
    X Basic operation
    - Convenience primitives (succeed, fail, execute, maybeDeferred)
    ? Callback/Errback chaining
    - Timeouts
- Composing deferreds
    - DeferredList/gatherResults
    - chainDeferred
- Advanced topics
    - Deferred asynchronous primitives
    - Sugar syntax

Cheers,
Edward

P.S. Please CC me in your replies! Thanks.