[v3] implement LWG 2067 and new issues with constructors in <future>

[Jonathan Wakely]

This patch implements
http://lwg.github.com/issues/lwg-active.html#2067 which has Ready
status, as well as fixing two new issues I've reported in the past few
hours.  The first is that packaged_task's template constructors should
be restricted to prevent them from being chosen to copy a
packaged_task object and the second is that promise and packaged_task
should properly support uses-allocator construction so that if
"promise(args...)" is well-formed then so is "promise(allocator_arg,
alloc, args...)"

        * include/std/future (promise): Add constructors for uses-allocator
        construction from rvalue promise.
        (packaged_task): Implement LWG 2067. Add additional constructors for
        uses-allocator construction.
        * testsuite/30_threads/packaged_task/cons/3.cc: New.
        * testsuite/30_threads/packaged_task/cons/alloc2.cc: New.
        * testsuite/30_threads/promise/cons/alloc2.cc: New.

Tested x86_64-linux, committed to trunk.
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