bfield 0.9

Convenient bit fields for int subclasses

# bfield.BitField

The missing bitfield type for Python 2 and 3.

## Example:

```
from bfield import BitField

class AX(int):
AL = BitField(0, 8, "The lower octet")
AH = BitField(8, 16, "The higher octet")


assert AX(0x1234).AH == 0x12
assert AX(0x1234).AL == 0x34
```

## Caveat

Note that due to specifics of immutable ints, read-only is the best thing
available. This is sufficient for decoding binary protocols and file formats.