BUG: revision of wilcoxon signed rank test. Closes ticket 1744.

Wilcoxon signed rank test does not comply with MASS R package for
some datasets.
Merge branch 'wilcoxon_fix' into master.

Reviewed as https://github.com/scipy/scipy/pull/338
ENH: optimize/minpack: determine leastsq and fsolve epsfcn by dtype (fixes Trac #920)

In particular, if the objective function returns float32, use a large
enough epsilon for gradient estimation.