Logbuf rewrite + specify number of log lines to return

Closes #133, closes #92.

Docker backend is untested, and logbuf event buffering until a newline is missing.
I just tried to do `flynn log ID` when booted under docker and it doesn't return anything, even though the attach was logged in the `flynn-host` log:

```
now=2014-10-05T17:36:28+0000 app=host at=start fn=attach job.id=27c85cf3938b41aba33a28b45987c87b
now=2014-10-05T17:36:28+0000 at=attach app=host fn=attach job.id=27c85cf3938b41aba33a28b45987c87b
now=2014-10-05T17:36:28+0000 app=host fn=attach job.id=27c85cf3938b41aba33a28b45987c87b at=success
now=2014-10-05T17:36:28+0000 app=host fn=attach job.id=27c85cf3938b41aba33a28b45987c87b at=finish
```