Improve distribution fitting as described in ticket #1553
http://projects.scipy.org/scipy/ticket/1553
Restored nnlf function and added _penalized_nnlf
Simplified the fit method.

Made sure fit_loc_scale method returns finite values in order to avoid failure in the fitting of the distributions
TST: stats: enable the distribution fitting tests
DOC: stats: add cautions to the fit method
STY: stats: in fit(), don't check for out-of-range points if allowed range is [-inf, inf]
TST: stats: turn off floating point errors in fit tests
Merge pull request 462 from pbrod/ticket1553

ENH: stats: Improve distribution fitting

Use a gradually penalized NNLF in MLE fitting, rather than one that returns inf
even if one point is outside the support of the distribtuon, confusing the
optimizers.