Implement bearing off

Crashes right now:

```
2012-12-31 15:07:44.431 Gammon[12841:c07]
1 2 3 4 5 6 7 8 9 10 11 12
X X X X X
X X X X
X X X
X X
X

O
O O
O O O
O O O O
O O O O O
24 23 22 21 20 19 18 17 16 15 14 13
Turn: Black
Roll: (
5,
2
)
Left: (
5,
2
)
2012-12-31 15:07:46.607 Gammon[12841:c07] pip tapped: 5
2012-12-31 15:07:46.607 Gammon[12841:c07] moving from 5 by 5
2012-12-31 15:07:46.608 Gammon[12841:c07] *** Assertion failure in -[Game moveFrom:by:], /Users/sas/Projects/Gammon/Gammon/Game.m:167
2012-12-31 15:07:46.611 Gammon[12841:c07] *** Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: 'from - by must be within [1, 24], was: 0'
*** First throw call stack:
(0x1c9c012 0x10d9e7e 0x1c9be78 0xb6ff35 0x8720 0x2a0c 0x5f84 0x2eb89a 0x2ea9db 0x2ec11f 0x2eed6d 0x2eecec 0x2e6a68 0x53fc2 0x544a3 0x323aa 0x23cf8 0x1bf7df9 0x1bf7ad0 0x1c11bf5 0x1c11962 0x1c42bb6 0x1c41f44 0x1c41e1b 0x1bf67e3 0x1bf6668 0x2165c 0x1dfd 0x1d25)
libc++abi.dylib: terminate called throwing an exception
```