Converted Database class to Multiton

I converted Database class to Multiton in order to stop polluting memory with `new \helpers\database();`.
`new \helpers\database();` isn't safe even with usage of reusing. This PR should fix bugs with Database class.

I also messed up during this PR, because .gitignore didn't affected app/core/config.php, so I had to remove cached version of it and put it back in place.
It's a fix to #83.

What are you thinking about it?