Feature to determine expected signal strength fit result from toys

Fixed version of #185

This adds a --max-likelihood-toys option to limit.py which runs toys and then max likelihood fits the toys, according to the prescription at [1]. The toys mu results are then produced in a TTree in the .root output.

The option respects the -s seed and --toys options.

[1] https://twiki.cern.ch/twiki/bin/viewauth/CMS/SWGuideHiggsAnalysisCombinedLimit#Toy_by_toy_diagnostics