Update FastCGI code and restructure headers a bit

For the headers I was a bit unsure how they should be structured, but I thought that since it was likely that log.h and fastcgi.h would probably be included in every file, I moved them to common.h. 

The FCGI_JSON has changed drastically (i.e any old code needs to be rewritten using the new functions), and you should be able to do everything without having to reside handler code within fastcgi.c