Added `CRADLE_ASSERT_PRECONDITION`.

* cradle/utility/assert_precondition.hpp: New. Added
  `CRADLE_ASSERT_PRECONDITION`.
* src/utility/jamfile: Added dependency on `/cradle/src/exception`.
* test/unit/utility/jamfile: Added copyright.
* test/unit/utility/as_const_run.cpp: Added copyright.
* test/unit/utility/assert_precondition_run.cpp: New. Added a unit test of
  `CRADLE_ASSERT_PRECONDITION`.
* test/unit/utility/simple_auto_forward_return_run.cpp: Added copyright.
Merge pull request #13 from Cryolite/feature/assert_precondition

Added `CRADLE_ASSERT_PRECONDITION`.