Convert functions to ES6 classes

Created by: Ginden

Most of these changes are whitespace changes.

I recommend running following command to see what was changed:

git clone git@github.com:Ginden/bull.git;
cd bull;
git remote add upstream git@github.com:OptimalBits/bull.git;
git fetch upstream;
git diff --ignore-all-space upstream/develop origin/chore/use-classes

I'm interested in converting project to TypeScript.