Support Trello

Trello API Documetation is [here](https://trello.com/docs/index.html).

There is no interface to add webhook on Trello, so I created [page](http://eitoball.github.io/add_webhook_to_trello_board/) to add hook easily.
oh... :open_mouth: orz...

Fixed and checked by running `bundle exec haml lib/hooks/trello/instructions.js.hbs.hamlbars | tidy`