this will return recent calls from unique client IPs.

In old code, when call history api enabled, if no one calls that api,
history record thunks will build up in memory. This problem is fixed
now.

this feature can be enabled by environment variable SERVE_HISTORY_PAGE=1

- add a Makefile
- allow specify listen port using env variables.

header for PHP is modified in some way like WSGI. they prefix it with
HTTP_ and replace '-' by '_'. In haskell, I just need the raw header.