I run a small website and my nginx access logs are piling up as gzipped files in a folder. Build me a command-line tool I can run tonight.

Point it at a folder of nginx access logs (the standard "combined" log format) and have it print a summary:
- total requests
- unique visitors (by IP address)
- the top 10 pages by number of hits
- the overall error rate (share of responses that are 4xx or 5xx)
- requests broken down by hour of the day

Two hard requirements: it has to handle big log files without loading the whole thing into memory (I have some large ones), and it must not crash on the occasional garbage line. Tell me exactly how to run it.
