Add Harmonia task URL to Trello webhook callback URL.
Use Mechanize to mark Harmonia tasks as done.

Added specs for handling POST requests to Trello events action.
Indent Sinatra app code prior to moving it into a class.

I think this will make it more obvious what's going on.
Move Sinatra application code into a WebhooksApp class.

This is so I can inject some dependencies into it.
Extract Harmonia class to encapsulate requests to Harmonia.
Move Harmonia class into its own file.
Capitalize 'head' HTTP method in spec name.
Added specs for unhappy paths for Trello events.
Extract Trello event logic into a TrelloEvent class.
Move TrelloEvent into a separate file.
Unit test TrelloEvent to avoid combinatorial explosion.
Move ancillary files into 'lib' directory.
Add comment to card when task is marked as done.