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.
Since this code is only investigating the existence of the constant, how is it that uses not `const_get` but `const_defined?`?