Complete rewrite of backend

* Backend rewritten in C++
* Posts are now directly scarp from 9GAG website instead of using the
unstable InfiniGAG API
* Vote section temporary remove because it's HTML is a bit different
* Image size temporary remove because the website only provide medium
size image url
Add a singleton Settings class in C++

To enable both C++ and QML side easily access to the settings and also
slightly improve performance
Bump version to 0.3.0 for release