Remove Timer::stop().  It does not fit with beginFrame/endFrame.
Add UnitTests for Timer class.
Remove Timer usage from ExampleBouncingBalls.

Trying to keep ExampleBouncingBalls uncluttered.
Normalize action tense for documentation comment in Timer class.
Use namespaces in Timer.cpp instead of explicit scope resolution.

Changed for consistency with other code.
Rename Timer::m_frames to m_frameDurations for clarity.
Use size_t instead of int for return values in Timer.
Remove while-loop waiting for non-failed clock time in Timer::now.

The boost::steady_clock::now() call can fail, but I have not seen it fail
in the Timer class.  The while loop looked like a hack with a
magic number.  Without the while loop we might get a bad time, but that
can be checked by getting the number of clock fails.
Add linebreaks to too-long lines caused by variable name change in Timer.
Add the Timer class's UnitTests in TimerTest.cpp to CMakeLists.
Remove commented-out #include line from TimerTest.cpp
Rename Timer::m_numberOfFrames to m_maxNumberOfFrames for clarity.
Fix copy/paste error in assert message about Timer not having frames.
Add function to get cumulative time from Timer class.

This is code that was already in getAverageFramePeriod but now one could
do something like:
Timer a_timer;
do_stuff_to_time();
a_timer.endFrame();
do_stuff_no_timing();
a_timer.beginFrame();
do_more_stuff_to_time();
a_timer.endFrame();
a_timer.getCumulativeTime(); // <- the sum of times for do_stuff_to_time
// and do_more_stuff_to_time, but not do_stuff_no_timing

Before this commit, that could be done with two timers, but this may
be cleaner.
Bring Timer.h documentation up-to-date with recent changes in Timer class.
Rename Timer::setNumberOfFrames to setMaxNumberOfFrames to match variable.
Consistently use Timer::beginFrame to start a new frame in Timer class.
Remove m_stopped from Timer class; no longer used anywhere.
Add Timer UnitTests for getCumulativeTime and when expecting asserts.
Make Timer.h documentation clearer and more explanatory.