In reply to slaFFik.

Very true. This is the current state of Composer being integrated into WP core: https://core.trac.wordpress.org/ticket/23912

Until Core wants to actually start using Composer as it's actual package manager (and it would be an amazing thing), the link that Dwain posted is about as good as it will get using Composer and WordPress. Which I use on a daily basis, and it works fine. WP just becomes a dependency itself, instead of the app that uses Composer. But that works too, it makes WP very portable and easy to make clean installs of.