Organize code into MVP or MVVM architecture.

[Skywalker97]

No need to add any functionality as such, just reorganize the code. :)

[vchernogorov]

I guess you should add CONTRIBUTING.md file first?

[codeteo]

Hi @Skywalker97 , I'm interested in working on this issue.

I've got two questions :

1. MVVM with data binding or with RxJava ? Or both depending on the case ?
2. After I quick lookup on the code I noticed nested adapter class, commented out code that is not removed and some other minor issues like code formatting. Can I also try to deal with these issues too in the same PR ?
3. Some drawables are missing from mipmap folder ( names : file, instorage and sdcard ). Could you, please, push a commit with the missing files cause I'm not able to build the project without them ?

Edit : added a 3rd question

@Skywalker97 maybe i can help with added some unit test