Trello service

This is a new PR for adding the outstanding Trello service hook contributed by @andreychernih in PR #35.  Expect the new service to be added shortly after some testing and minor adjustments!
When possible, I prefer to use xhtml compatible HTML markup, which would be
```
<textarea readonly="true">
   ... stuff
</textarea>
```

I don't have a great concrete reason other than that if we ever subject our html to some kind of validation it tends to have an easier time with well-formed XHTML, so up to you whether you want to take the steps to introduce this change.