Add system.multicall support for the XML-RPC server

New submission from ramen <None>:

The Twisted XML-RPC server does not come with system.multicall, a de facto 
standard method which allows a batch of methods to be called in a single 
request.

I wrote an implementation which handles deferred results and ensures that 
exceptions in one method call do not prevent the other methods from 
executing.

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Type     : enhancement
Component: core
Keywords : 
Priority : normal
Nosy     : 
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http://twistedmatrix.com/trac/ticket/3823