change constructor prototype to allow const pin arrays

It's annoying to not be able to declare the rows and cols pins or the keys as const without having a compile time warning.

Please change the constructor prototype to document the fact that you are not messing around with the arrays and thus they can be const.

please go for:

`Keypad::Keypad(const char *userKeymap, const byte *row, const byte *col, byte numRows, byte numCols)`
