Add a serial communication interface

[mvirkkunen]

MFRC522 supports I2C and serial UART besides just SPI, this adds support for the UART protocol.

On a microcontroller there probably isn't much advantage to using a UART over SPI, but this enables the module to be used on anything with a serial port or USB port with a USB to serial adapter. I included an example that should run on most computers. I'd like to add the anti-collision loop and support for the MIFARE stuff into this crate, and having a serial interface makes for a nicer development/debug cycle because I can run everything on a normal computer.

N.b. this currently changes API as I changed Mfrc522::new to take an interface object instead of just SPI. There are also helper methods new_spi and new_serial so that's it's equally nice to use.