Correct or improve C++11 and higher allocator support

[glenfe]

To correctly support the C++ allocator model, or otherwise simplify allocator-aware code.

1. 1. When the C++11 allocator model is available (i.e. BOND_NO_CXX11_ALLOCATOR is not defined) to rebind an allocator type A for type T, should always use std::allocator_traits<A>::template rebind_alloc<T> as I did in the recent Improvements and fixes for C++ nullable implementation and reduce dynamic allocation  #174 nullable changes. This is because a valid C++ allocator is no longer required to provide a rebind member template.
2. 2. The integral constant expression for the detection of the Default Allocator can be simplified by a writing trait is_default_allocator<A> that is the integral constant true_type when the given allocator is the Default Allocator, and false_type otherwise.