simple copy&paste

What I felt was missing while playing with Flowhub was the lack of copy-paste. So I "hacked" a simple implementation that does the following.

- copies one node
- copies a selection. When copying a selection it also duplicates the edges that are contained in the selection

Only works withing the same document due to Clipboard browser restriction

if you think it has a future :) let me know and I can refine if needed.