Made directories and sign up work

The directory name is now changed to 'site' as 'main' is also the name
of the script executed.

Also, I made the sign up process work, finally! How it works:
1. Sends Ajax request on submit button
2. Server processes inputs
3. Checks if username available
4. If not available, invoke 500 server error, jQuery will then know and
send error message
5. If available, just redirect to the next page