Removed security header

As we've discussed this before in #473, the security header like
```php
defined('SYSPATH') OR die('No direct access allowed.');
```
is no longer needed.

Instead this, we have an example of config for web servers:

__Nginx__ (Full config: https://github.com/gleez/cms/wiki/Nginx)
```nginx
server {

  # Blocks Direct Access To these directories
  location ~ /(application|modules|themes|system)/ {
    deny all;
  }

  # ROUTING TO GLEEZ IF REQUIRED
  location / {
      index index.php;
      try_files $uri $uri/ @gleez;
  }

  # HANDLES THE REWRITTEN URLS TO GLEEZ CONTROLLER
  location @gleez {
      rewrite / /index.php;
  }

}
```

__Apache__ (Full config: https://github.com/gleez/cms/wiki/Apache)
```apache
# Start rewrite engine
<IfModule mod_rewrite.c>
	# If your web host doesn't allow the FollowSymlinks option, you may need to
	# comment it out and use `Options +SymLinksIfOwnerMatch`, but be aware of the
	# performance impact: http://goo.gl/Mluzd
	Options +FollowSymlinks
	# Options +SymLinksIfOwnerMatch

	# Turn on URL rewriting
	RewriteEngine On

	# Installation directory
	RewriteBase /

	RewriteCond %{THE_REQUEST} ^.*/index.php
	RewriteRule ^(.*)index.php$ /$1 [R=301,L]

	# Protect application and system files from being viewed
	RewriteRule ^(?:application|modules|themes|system)\b.* index.php/$0 [L]

	# Allow any files or directories that exist to be displayed directly
	RewriteCond %{REQUEST_FILENAME} !-f
	RewriteCond %{REQUEST_FILENAME} !-d

	# Rewrite all other URLs to index.php/URL
	RewriteRule .* index.php/$0 [PT]
</IfModule>
```


For other servers such tuning should not be a problem.  None of the current modern CMF/CMS does not have such security header: Symfony, Zend Framework, Yii, Drupal, Wordpress