octal parsing

Data.Attoparsec.ByteString.Char8 has decimal and hexadecimal, but no octal. Since octals are used for file permissions, it would be good to have a ready made parser for them. In my case I'm parsing git ls-tree output, which includes file permissions.

It's easy enough to adapt the decimal and hexadecimal code into the following.
I mostly submitted this because now, having modified your code, I have to include the 3 clause BSD license in my program in order to properly document your copyright, and would rather send it upstream. ;)

```
-- | Parse and decode an unsigned octal number.
--
-- This parser does not accept a leading @\"0o\"@ string.
octal :: Integral a => A.Parser a
octal = B8.foldl' step 0 `fmap` I.takeWhile1 isOctDigit
where
isOctDigit w = w >= 48 && w <= 55
step a w = a * 8 + fromIntegral (w - 48)
```