added setRowOffsets to liquid crystal library

There are several LCD displays that have different internal memory foot prints than the standard library supports.  For example, the normal row offsets are {0x00, 0x40, 0x14, 0x54}, other displays need different offsets such as { 0x00, 0x40, 0x10, 0x50 };

This change keeps the default the way it is and adds a method setRowOffsets which is called as follows

     LCD.setRowOffsets(0x00, 0x40, 0x10, 0x50);

No impact on existing code and it simply moves the array definition from the setCursor function into the class definition.