Rewrite deferred documentation

New submission from ezyang <None>:

There seems to be a general consensus (on #twisted at least), that the old documentation on Deferreds kind of sucks. The planned new outline is:

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

One of the ideas behind the new documentation is that there is a mapping between synchronous code and asynchronous code, and as such copious examples to explain this are to be provided. In particular, errbacks are simply a userland reimplementation of exception handling.

I expect the hardest pieces to explain will be:

 1. The asynchronous paradigm shift
 2. Callback/Errback chaining (in which the method names are totally misleading; I'm going to take a "please ignore the abstraction barrier" approach to this one)
 3. Deferred composition

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Type     : enhancement
Component: core
Keywords : 
Priority : normal
Nosy     : 
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http://twistedmatrix.com/trac/ticket/3943