doctests for byteAt

````irc
01:07 <lensbot> [lens] ekmett pushed 1 new commit to master:
      http://git.io/GuhGcQ
01:07 <lensbot> lens/master f6e6c22 startling: byteAt
01:08 <startling> :D
01:08 <startling> I was gonna say let me write some unit tests first, but
01:08 <edwardk> feel free to send some of those in too. doctests for something
      like this are really handy
01:09 <startling> will do
````