Fix a couple of warnings under gcc 4.7.2

During compilation using `gcc 4.7.2` compiler we noticed a couple of warnings.
1. `-Woverloaded-virtual`: it warns about functions that might not be overloaded because of a typo in their signature
2. Unsigned variables cannot be less than zero.
3. Unused parameters produce warnings.

Also we add some `const` to `intersects` methods, because they are constant.