Use time.perf_counter when available

Created by: nthykier

As of python3.3, the recommended method for obtaining a performance timer is no longer "time.clock()" but "time.perf_counter()". By using time.perf_counter, we outsource the problem for finding the best performance messaurement to python. Plus, we also avoid inaccurate timings if the system time is changed during the processing of the SVG image on platforms where we used to use "time.time()".

For python2 and python3 before 3.3, we still have to use the original timers.

Signed-off-by: Niels Thykier niels@thykier.net