add a test to avoid a division by zero

When the cacheplugin is used in guzzle, the total time
of the request is zero.

So when calculating the percents in the debug bar there is
an exception :

	An exception has been thrown during the rendering of a template
	("Warning: Division by zero in FILE) in
	PlaybloomGuzzleBundle:Profiler:request.html.twig at line 15

To avoid this we do not calculate to percents nor display to
time table (witch is irrelevant in this case).
Merge pull request #18 from agallou/notice_zero

add a test to avoid a division by zero