Improve logging

[kythyria]

The log writing is fairly dire at the moment, being little more than a dump of what gets appended to the scrollback.

So, to improve it:

• Log rotation, preferably with a choice of when to rotate (size, date)
• A heuristic to avoid rotating mid-session (unless the session is really long)
• Plain text logs
• Logs in (nearly) well-formed HTML with a real stylesheet and everything.