Design document for xylem

 - includes motivation and goals from initial meeting
 - includes discussion of which types of plugins / entry points xylem should support
 - includes a lot of concrete suggestions on improvements over rosdep
 - is meant to evolve following discussions and new realizations

You can find a rendered version here: http://xylem.readthedocs.org/en/design/design.html

@wjwwood: I would kindly as for feedback on my explanations on motivation, goals, etc. Especially the section on plugins and improvements over rosdep contains many open design questions that should be discussed at some point, but maybe the first thing to do is to assign priorities and then flesh-out / prototype the most important parts before getting lost in too much abstract discussion.