http.c v1.04 , net.c v1.14 (response head)

Add response head:
version,
date,
server,
last modified time,
content type,
content length.
main.c v1.05(rebuilt filecheck())

In order to record log, we change the filecheck function.
Also, it gives a warning when log file exist and is not a regular file.
net.c v1.15(logging)

Add logging:
format: ’%a %t "%r" %>s %b’
%a The remote IP address.
%t The time the request was received (in GMT).
%r The (quoted) first line of the request.
%>s The status of the request.
%b Size of the response in bytes. Ie, "Content-Length"
http.c v1.05(logging)

Calculate global values to log.
add argument l_file
net.c v1.15(logging)

Give simple response a appropriate status.
Merge pull request #37 from lzhang25/master

main.c v1.05, net.c v1.15, http.c v1.05 (logging & response head)