Improve documentation

[timiwahalahti]

We need to improve inline documentation and also open a GitHub wiki that documents every hook properly.

[anttiviljami]

I'm against using Github wiki for documentation. Add API.md or something instead so we can keep docs in source control.

[k1sul1]

Just add docblocks and autogenerate the docs, publish on ghpages.

One addition to composer.json:

"phpdoc": "phpdoc -n -t docs -d modules --template=\"responsive-twig\"; git add docs; git commit -m \"Update documentation\""

[timiwahalahti]

At the same time, let's introduce better changelog as keepachangelog.com that contains also Unreleased section.