Remove unused Callback feature from solvers
Small optimizations
Replace C-style headers with Cpp-style headers
Rename Board::usedCells to Board::count
[1/4] Add "Only this cell can be X" verification to BillSolver

This first commit only looks through the rows (or columns, depending on your POV =P)
[2/4] Add "Only this cell can be X" verification to BillSolver

Now it also checks if it's the only possibility on the column (or row, depending on your POV =P)