Huboard.com integration

I've integrated the harvest button into the kanban style board offered by huboard.com (it takes the issues from github and presents them in a more organized way). In the process, i've switched the strategy for adding the button to an interval based one. This was needed since on GitHub you would lose the button if you have updated the issue (just change the assignee and the button disappears) and similar effects on huboard.com. I couldn't find any sensible events to attach to so the interval was needed.