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.
Merge pull request #427 from pv/bug/920-float32-leastsq

ENH: optimize/minpack: determine leastsq and fsolve epsfcn by dtype (fixes Trac #920)