Complete comment functionality

This implementation was created in accordance with the first case variation for node comments as it was described in issue #329
![basic variations](https://f.cloud.github.com/assets/3720963/613204/5b039260-cdfc-11e2-969c-197e53c2bc55.png)
https://github.com/nodebox/nodebox/issues/329#issuecomment-16788843