Improved documentation...

Improved documentation...

... of examples/example.cpp, examples/example_bipgraph.cpp,
include/dai/prob.h, include/dai/smallset.h, include/dai/var.h,
include/dai/varset.h

Some small additional changes:
- Replaced TProb<T>::log0() by TProb<T>::log(true)
  and TProb<T>::takeLog0() by TProb<T>::takeLog(true)
- Replaced TFactor<T>::log0() by TFactor<T>::log(true)
- Removed TProb<T>::hasNonPositives()
- Added examples/example_varset.cpp
- Renamed smallSet<T> to SmallSet<T>
- Changed operator<< for Var and VarSet