Added support for the SPI bus

I bought a MFRC522 RFID reader and have managed to get it working with node-firmata.  I found a [working sketch](http://www.grantgibson.co.uk/2012/04/how-to-get-started-with-the-mifare-mf522-an-and-arduino/) that talks to the card over the SPI bus which I ported to JavaScript.

I looked at the [SPI proposal on the wiki](http://firmata.org/wiki/Proposals#SPI_Proposal) and cross referenced it with the [Arduino API](http://arduino.cc/en/Reference/SPI) - please accept my apologies if I've got the wrong end of the stick, but I can't see how one applies to the other - the proposal on the Wiki page refers to a scheduler, creating jobs, querying jobs, deleting jobs and so on, but the [SPI class](https://github.com/arduino/Arduino/blob/master/libraries/SPI/SPI.h) is all about reading and writing bytes to devices on the SPI bus.

Anyway the class defined in `SPI.h` look pretty straightforward so I had a go at exposing it's methods.  The protocol looks like this:

```
/* SPI transfer request
 * -------------------------------
 * 0  START_SYSEX (0xF0) (MIDI System Exclusive)
 * 1  SPI_REQUEST (0x60)
 * 2  SPI_TRANSFER (0x00)
 * 3  sequenceId (byte) Used to match requests to responses
 * 4  toWrite (byte) How many bytes to write onto the bus
 * 5  data 0 (byte)
 * 6  data 1 (byte)
 * ...
 * n  END_SYSEX (0xF7)
 */
```

```
/* SPI transfer reply
 * -------------------------------
 * 0  START_SYSEX (0xF0) (MIDI System Exclusive)
 * 1  SPI_REQUEST (0x60)
 * 2  SPI_TRANSFER_REPLY (0x01)
 * 3  sequenceId (byte) Used to match responses to requests
 * 4  SPI_READ (0x02)
 * 5  read (byte) How many bytes were read from the bus
 * 6  data 0 (byte)
 * 7  data 1 (byte)
 * ...
 * n  END_SYSEX (0xF7)
 */
```

```
/* Begin SPI request
 * -------------------------------
 * 0  START_SYSEX (0xF0) (MIDI System Exclusive)
 * 1  SPI_REQUEST (0x6W0)
 * 2  SPI_BEGIN (0x02)
 * 3  END_SYSEX (0xF7)
 */
```

```
/* End SPI request
 * -------------------------------
 * 0  START_SYSEX (0xF0) (MIDI System Exclusive)
 * 1  SPI_REQUEST (0x60)
 * 2  SPI_END (0x03)
 * 3  END_SYSEX (0xF7)
 */
```

```
/* Set bit order
 * -------------------------------
 * 0  START_SYSEX (0xF0) (MIDI System Exclusive)
 * 1  SPI_REQUEST (0x60)
 * 2  SPI_SET_BIT_ORDER (0x04)
 * 3  bitOrder (byte) Sets the order of the bits shifted out of and into the
        SPI bus, either LSBFIRST (0 - least-significant bit first) or MSBFIRST
        (1 - most-significant bit first).
 * 4  END_SYSEX (0xF7)
 */
```

```
/* Set data mode
 * -------------------------------
 * 0  START_SYSEX (0xF0) (MIDI System Exclusive)
 * 1  SPI_REQUEST (0x60)
 * 2  SPI_SET_DATA_MODE (0x05)
 * 3  dataMode (byte) Sets the SPI data mode: that is, clock polarity and phase.
 * 4  END_SYSEX (0xF7)
 */
```

```
/* Set clock divider
 * -------------------------------
 * 0  START_SYSEX (0xF0) (MIDI System Exclusive)
 * 1  SPI_REQUEST (0x60)
 * 2  SPI_SET_CLOCK_DIVIDER (0x06)
 * 3  clockDivider (byte) Sets the SPI clock divider relative to the system
        clock. The dividers available are 2, 4, 8, 16, 32, 64, or 128. The
        default setting is SPI_CLOCK_DIV4, which sets the SPI clock to
        one-quarter the frequency of the system clock.
 * 4  END_SYSEX (0xF7)
 */
```

```
/* Attach interrupt
 * -------------------------------
 * 0  START_SYSEX (0xF0) (MIDI System Exclusive)
 * 1  SPI_REQUEST (0x60)
 * 2  SPI_ATTACH_INTERRUPT (0x07)
 * 3  END_SYSEX (0xF7)
 */
```

```
/* Detach interrupt
 * -------------------------------
 * 0  START_SYSEX (0xF0) (MIDI System Exclusive)
 * 1  SPI_REQUEST (0x60)
 * 2  SPI_DETACH_INTERRUPT (0x08)
 * 3  END_SYSEX (0xF7)
 */
```

It'd be great if this could be added to ConfigurableFirmata.

I don't know if all the messages are strictly necessary (setting the clock divider for one seems specific to Arduino), but I'd love to have your input - please let me know what you think.