Issue #413: Use Symfony var_dumper() function instead of Kint.

http://symfony.com/doc/current/components/var_dumper/

* new dump() function, outputs structured elements to the SAPI (the web page if called from the web, the console if called from a script)
* helper function cli_dump() added, allows outputing debug info to the CLI from a web page
* new app/helpers folder for such functions
* app/helpers/debug.php autoloaded via composer