Add conf.int to wilcox test.

[rwjblue]

Requested by @krisselden after a round of testing, and seemed generally useful to have:

this shows you the range it is 95% confident the location shift was within

Given:

    Wilcoxon rank sum test with continuity correction

data:  ms by set
W = 3499, p-value = 0.009923
alternative hypothesis: true location shift is not equal to 0
95 percent confidence interval:
  4.185039 30.244932
sample estimates:
difference in location
              17.92599

95% chance the shift was between 4ms and 30ms and 18ms median