Small fixes; fix use outside of CMSSW

[swertz]

Small fixes: -The field plot_cut in the plots variable in the python config had become mandatory when it shouldn't be

• The fields is-data and db-name in the sample JSONS do not have to be mandatory

It's now possible to use HistFactory and TreeFactory completely outside of CMSSW. Only thing that is needed is Seb's TreeWrapper library, and some tweaks to link to the latter when running createPlotter (see README).

[blinkseb]

Looks good, thanks a lot for the doc!!