Added support for RedBearLab nRF51822 Board

Hey,

I added support for the nRF51822 Board. With a few exceptions it behaves similar to the Arduino Due, which is why I extended its DEFINEs whereever I could.
I wasn't able to run the GPIO test due to a lack of time and breadboard.


board_qc Output:

 $$$109002032C272E6C1341AFC1 (<- caused by the MK2 Bootloader)
Circuits At Home 2011
USB Host Shield Quality Control Routine
Reading REVISION register... Die revision 02
SPI long test. Transfers 1MB of data. Each dot is 64K................ SPI long test passed
GPIO test. Connect GPIN0 to GPOUT7, GPIN1 to GPOUT6, and so on
Test failed. Value written: 00 Value read: FF 
Press any key to continue...
GPIO test passed.
PLL test. 100 chip resets will be performed
Resetting oscillator

Reset number 0 Time to stabilize - 117 cycles
Reset number 1 Time to stabilize - 117 cycles

[...]

Checking USB device communication.

Reset complete. Waiting for the first SOF...
Getting device descriptor
Descriptor Length:	12
Descriptor type:	01
USB version:		0110
Device class:		00
Device Subclass:	00
Device Protocol:	00
Max.packet size:	10
Vendor  ID:		09E8
Product ID:		007C
Revision ID:		0100
Mfg.string index:	01
Prod.string index:	02
Serial number index:	00
Number of conf.:	01

All tests passed. Press RESET to restart test
Can you move this to here instead: https://github.com/felis/USB_Host_Shield_2.0/blob/master/settings.h#L141?