Half-duplex UART using the USI module for the attiny85, based on Atmel application note AVR307.

Note that this has not been tested yet! I cannot promise that it will work perfectly.

See "echo.c" for an example program that implements a simple echo function.

See "usi_uart.h" for constant definitions & prototypes. A brief description:

> reverse_byte
    - reverses a single byte in O(1) time with a lookup table

> uuart_flush_buffers
    - clears the rx and tx buffers

> uuart_init_receiver
    - initializes the receiver driver

> uuart_rx_byte
    - receives a single byte

> uuart_data_in_rx_buffer
    - returns false if the receive buffer is empty

> uuart_init_transmitter
    - initializes the transmitter driver. called by uuart_tx_byte, so you
      don't need to call it yourself.

> uuart_tx_byte
    - sends a single byte