Release 2015.12 feature freeze

Hey fellow RIOTers,

we're now officially in feature freeze for our upcoming quarterly release (2015.12).

That means: * branch 2015.12-branch is the latest release branch, from now on it will only get bug fixes

* tag 2015.09-last marks the last bugfixing commit for the 2015.09 release

* tag 2016.03-devel marks the start of working towards the next release and is mostly needed for the version string

* pull requests for non-2015.12-stuff might get delayed reviews/comments, please understand

* help testing 2015.12 is highly appreciated!

We plan on tagging the final 2015.12 release on Friday next week (December 18th).

Happy breaking!

Cheers, Kaspar