Some change in recent GCC (4.7.2) caused this to fail building, now
using duration_cast<microseconds>() instead of microseconds(). Also
cleaned up the code by importing std::duration prefix.
Optimalizations in Corrade::TestSuite and Corrade::Utility::Debug leaded
to significant reduction of compilation time - on my machine it was
~5:38 before with building of unit tests enabled, now only ~5:00.