Unordered: Merge unordered from trunk.

- Activate `std::allocator_traits` for gcc 4.7 and Visual C++ 11.
- Implement variadic construct in `boost::unordered::detail::allocator_traits`
  when variadics, rvalue references and SFINAE expression are available.
- Use variadic construct from `allocator_traits`, or when not available move
  the logic for constructing `value_type` to a lower level, so the container
  code is a bit simpler.
- Avoid `-Wshadow` warnings. Fixes #6190.
- Implement `reserve`. Fixes #6857.


[SVN r78432]

test/unordered/bucket_tests.cpp

@@ -21,7 +21,7 @@
 
 namespace bucket_tests {
 
-test::seed_t seed(54635);
+test::seed_t initialize_seed(54635);
 
 template <class X>
 void tests(X* = 0, test::random_generator generator = test::default_generator)