Removed threadpool API again as the server will be single-threaded.
Added python script for generating http.h, a file containing HTTP responses for the HTTP error codes with a corresponding HTML file in sites/.
Also added HTML files for:
 - 400 Bad Request
 - 403 Forbidden
 - 404 Not Found
 - 405 Method Not Allowed
 - 408 Request Timeout
 - 431 Request Header Fields Too Large
 - 503 Service Unavailable
Added HTTP server implementation.