Refactored init.php to remove global path initialization

Since initialization is already being done in Loader class, we should not set THINKUP_ROOT_PATH and THINKUP_WEBAPP_PATH in any other place.

I also found that setting THINKUP_WEBAPP_PATH to THINKUP_ROOT_PATH . 'thinkup/' is incorrect since there is no such directory. This bug might not have been exposed till now because init.php sets the path and hence the Loader class code never executes.