Factor Graph

@ppletscher, @iglesias, @vigsterkr:
I added basically 3 stuff: FactorType, Factor and FactorGraph. I also create a new label class FactorGraphLabels which is similar as SequenceLabels. 

Sorry for the late PR. Actually it's not complete as many unit-tests need to be included. I'd like you to comment first and I'll finish them as soon as possible. Next step I'll do FactorGraphModel and try to estimate parameters by PrimalMosekSSVM.