C++ run-time timers

This is a set of C++ stopwatch classes for gathering
statistical data on the run-time performance of client code.
The classes will accumulate data on the time taken
for each start/stop interval, and report parameters
such as the minimum, maximum and average time
after the timer goes out of scope.

Features:

• Header only
• Templated to allow customization of clock source, timing statistics, etc.
• C++03 and C++11 (std::chrono) support
• Scope-based or manual starting/stopping
• Thread-safe and serial options
• Numerically stable calculation of mean & standard-deviation statistics
• Automatic scaling to nanosecond/microsecond/minute/etc. units

Example usage

#include <rtimers/cxx11.hpp>
void expensiveFunction() {
    static rtimers::cxx11::DefaultTimer timer("expensive");
    auto scopedStartStop = timer.scopedStart();
    // Do something costly...
}

When the program terminates, the timer will send a report
of the following form to std:cerr

Timer(expensive): <t> = 8.867us, std = 3.463us, 4.263us <= t <= 57.62us (n=731)

which shows the average time spent within expensiveFunction(),
its standard deviation, the upper and lower bounds,
and the total number of calls.

On POSIX systems with older versions of C++, one could use:

#include <rtimers/posix.hpp>
rtimers::posix::DefaultTimer timer("bottleneck");

Or for systems on which the Boost
libraries are available, including
the boost::posix_time datastructures:

#include <rtimers/boost.hpp>
rtimers::boostpt::DefaultTimer timer("bottleneck");

For multi-threaded code, one can use the following timer classes:

rtimers::cxx11::ThreadedTimer
rtimers::posix::ThreadedTimer
rtimers::boostpt::ThreadedTimer

Preprocessor macros are available for the combined declaration
of a static timer instance and a scoped start+stop:

RTIMERS_BOOST_STATIC_SCOPED(name)
RTIMERS_CXX11_STATIC_SCOPED(name)
RTIMERS_POSIX_STATIC_SCOPED(name)
RTIMERS_STATIC_SCOPED(name)

More specialized timers can be built by combining components
such as rtimers::HiResClock, rtimers::SerialManager,
rtimers::MeanBoundStats, rtimers::LogBoundStats,
rtimers::StreamLogger, etc. as illustrated in the
supplied demo.cpp.