Added `cradle::suppress_unused_variable_warning`.

* cradle/utility/suppress_unused_variable_warning.hpp: Newly added.
* test/unit/functional/invoke_run.cpp: Removed the definition of
  `suppress_unused_variable_warning`.
Merge pull request #24 from Cryolite/feature/addSuppressUnusedVariableWarning

Added `cradle::suppress_unused_variable_warning`.