MINOR: lua: allow socket api settimeout to accept integers, float, and doubles

MINOR: lua: allow socket api settimeout to accept integers, float, and doubles

Instead of hlua_socket_settimeout() accepting only integers, allow user
to specify float and double as well. Convert to milliseconds much like
cli_parse_set_timeout but also sanity check the value.

http://w3.impa.br/~diego/software/luasocket/tcp.html#settimeout

T. Fournier edit:

The main goal is to keep compatibility with the LuaSocket API. This
API only accept seconds, so using a float to specify milliseconds is
an acceptable way.

Update doc.