Move master to 2.0

This PR moves master to 2.0@dev. As there is a 1.2 branch, that should not be a problem for anyone ... except if you target dev-master in your composer.json, but hey, that's your choice.

Of course, it's not finished yet, but working on master is easier for everyone and future PRs.

TODO:

 - [ ] update documentation
 - [ ] update CHANGELOG