Stack<>: add _ suffix to member ownAllocator_

For consistency, all member variables should follow the same pattern.
MemoryPoolAllocator, Stack: lazily allocate Allocators

In order to make the constructors more efficient, especially
in the context of C++11 move semantics, the (dynamic) allocations
in MemoryPoolAllocator and Stack should be performed lazily.

Move the allocations to the first use of the allocator in both
classes.
Merge pull request #188 from pah/feature/lazy-alloc-allocators

MemoryPoolAllocator, Stack: lazily allocate Allocators