This removes the timer class. The technique of measuring the time
difference between two different OpenCL markers on a command queue
is not portable to all OpenCL implementations (only works on NVIDIA).
A new internal timer class has been added which uses boost::chrono
(or std::chrono if BOOST_COMPUTE_TIMER_USE_STD_CHRONO is defined).
This new timer is used by the benchmarks to measure time elapsed
on the host.
This cleans up the example code. Now all of the examples use
the "namespace compute = boost::compute" alias. This shortens
the example code making it less verbose and more clear. Also
cleans up a few style issues.
