[In progress] Code style fixes according to style guide mentioned in CONTRIBUTING.md

There are two big things left: function invocations and object literals, but I want to show what is already done and check if I'm going in the right direction at all, so would appreciate any thoughts/comments. 

Honestly, I decided to work on this in the first place just because I wanted to became more familiar with existing code. Right now it feels like I did. And the changes are just a bonus to that knowledge. :)

I tried to make as granular commits as I could – in theory there is no problem to edit or just strip out one commit or more if it's wrong or inappropriate.

And yeah, git shows a lot of changes (796 additions and 844 deletions right now) but they all are only about code style (like replacing `&&` with `and`), reviewing them shouldn't be the problem *(I hope)*.