0.1.2 release

• improved Doxygen documentation
• improved const correctness for getters and parameters
• replaced NULL with nullptr
• replaced if/else chains with switch statements
• added explicit SPI_KEYPAD_INVALID enum value
• improved constructor initialization using initializer lists
• improved const correctness for getters and parameters
• verified compatibility with C++11
• fix select and address parameter to uint8_t
• add setSPIspeed implementation
• fix examples to compile without warning
• minor edits