Merge pull request #600 from Q-Leap-Networks/pull-zdlist

Create new class for doubly linked lists.
New code wasn't working with ZMQ v2
Merge pull request #606 from hintjens/master

New code wasn't working with ZMQ v2
zdlist: Indentation: convert tabs to 8 chars
zdlist: add spaces before (
zdlist: blank line before method declaration
Merge branch 'master' into debian
Merge branch 'pull-zdlist-style-fix' into debian