Added basic driver example use code to the library documentation.

Made the USARTStream global public and documented in the SerialStream module, allowing for the serial USART stream to be accessed via its handle rather than via the implicit stdout and stdin streams.

LUFA/Drivers/Peripheral/SPI.h

@@ -45,6 +45,26 @@
  *  Driver for the hardware SPI port available on most AVR models. This module provides
  *  an easy to use driver for the setup of and transfer of data over the AVR's SPI port.
  *
+ *  <b>Example Usage:</b>
+ *  \code
+ *      // Initialise the SPI driver before first use
+ *      SPI_Init(SPI_SPEED_FCPU_DIV_2 | SPI_ORDER_MSB_FIRST | SPI_SCK_LEAD_FALLING |
+ *               SPI_SAMPLE_TRAILING | SPI_MODE_MASTER);
+ *
+ *      // Send several bytes, ignoring the returned data
+ *      SPI_SendByte(0x01);
+ *      SPI_SendByte(0x02);
+ *      SPI_SendByte(0x03);
+ *
+ *      // Receive several bytes, sending a dummy 0x00 byte each time
+ *      uint8_t Byte1 = SPI_ReceiveByte();
+ *      uint8_t Byte2 = SPI_ReceiveByte();
+ *      uint8_t Byte3 = SPI_ReceiveByte();
+ *
+ *      // Send a byte, and store the received byte from the same transaction
+ *      uint8_t ResponseByte = SPI_TransferByte(0xDC);
+ *  \endcode
+ * 
  *  @{
  */