modular API, bug fix for C4.5

- modular API setup for C4.5
- API example added
- 'loss of subsets of input feature matrix' fixed
@iglesias I am not sure but I think this include is for swig to make sense of ```TreeMachine<C45TreeNodeData>``` used in multiclass.i . Its not for exposing this class to the API. Is that the case? I actually don't have much idea about swig.