Merge pull request #426 from ndawe/master

[MRG] README rewording
Merge pull request #427 from ndawe/master

[MRG] add __repr__ for histfactory objects
Merge pull request #430 from ndawe/master

[MRG] RooWorkspace::import -> __call__
Merge pull request #434 from ndawe/master

[MRG] subclass more graph classes