Added `cradle::enable_if_reference`.

* cradle/type_traits/enable_if_reference.hpp: New. Added
  `cradle::enable_if_reference` and `cradle::disable_if_reference`.
* test/unit/type_traits/enable_if_reference_run.cpp: New. Added unit tests
  for `cradle::enable_if_reference` and `cradle::disable_if_reference`.
Merge pull request #15 from Cryolite/feature/enable_if_reference

Added `cradle::enable_if_reference`.