add basic comment node type
show comment on mouse over comment nodes
user node parameter setter in runtime
display comment next to comment node

either right, if there's enough space, or left of the node
until paperjs supports AreaText, a more beautiful solution is a lot of work,
since we'd have to compute the space occupied by the text manually.
always render comment text of comment node
comment nodes only consist of the comment itself

also, yellow background, preserve select and drag functionality
fix empty comments, comment d&d
prevent comment jumping
Merge pull request #39 from Doik/comments

Comment nodes!