Added Print::print(const Printable*) and Print::println(const Printable*)

Print::print(*ptr) may cause some problems if ptr is NULL.
In addition this solves a problem having a generic class A<T> extending Printable. If A<T>::printTo tries to print A<T>::t of type T a call to Print::print(T) will be ambiguous for pointer types. Dereferencing A<T>::t would break support for non-pointer types.