1st attempt to fix #50 

If user did not fill all fields, he gets a popup and is redirected to the register page
Testing purpose
Added stars for mandatory fields
Added include_once
Removed testing stuff
Update index.php
Debugging
Fixed various bugs
Added error message
Fogotten ; added
Fixed unnecessary typo
Fixed error message
changed path of redirect
Update register.php
Altered redirect
replaced warning
Update register.php
Fixed session_destroy(); bug
used include_once();
added session