Added `cradle::polymorphic_allocator`.

* jamroot: Added copyright. Added an alias to `src/memory`.
* cradle/memory/polymorphic_allocator.hpp: New. Added interfaces of
  `cradle::polymorphic_allocator`.
* src/memory/jamfile: New.
* src/memory/polymorphic_allocator.cpp: New. Added implementation of
  `cradle::polymorphic_allocator`.
* test/unit/memory/jamfile: New.
* test/unit/memory/polymorphic_allocator_run.cpp: New. Added unit tests of
  `cradle::polymorphic_allocator`.
Reduced the number of `BOOST_CHECK_*`.

* test/unit/memory/polymorphic_allocator_run.cpp: Reduced the number of
  `BOOST_CHECK_*` because many `BOOST_CHECK_*` place an unnecessary burden
  on unit test process. In addition, many `BOOST_CHECK_*` lead to large XML
  reports, and Jenkins xUnit plug-in refuses such reports.
Merge pull request #16 from Cryolite/feature/polymorphic_allocator

Added `cradle::polymorphic_allocator`.