Enhancement : Test for significance of execution time differences

[jeromepcollet]

Add significance test of time difference

In the summary of a benchmark, it would be useful to have a test of significance of the time differences.
It is possible with the following code :
kruskal.test(time ~ expr, data = bench)
I propose Kruskal test because it works for any number of groups.

[tinygreen]

Try the already implemented Tukey test which is included in the output if the multcomp package is installed. Note issue #43.

[jay-sf]

I think this Stack Overflow answer shows that the cld column actually shows groups with no significant differences (rather than a ranking).