Dropbox watch agent

This agent implements a very simple way of detecting file changes in Dropbox.

After fiddling around a little bit on PetroFeed@c947099, we came to the realization that we would not be able to do all things we want to achieve on a single agent. Well, we could, if we were to write a heavily customized Agent that no one else could benefit of. So I'm seeing this PR as the beginning of a collection of Dropbox agents, addressing #321.

Here's a workflow we'd like to be able to achieve:
- Watch an email inbox for a specific email with attachment – (maybe using an evolution of the [ImapFolderAgent](https://github.com/cantino/huginn/blob/master/app/models/agents/imap_folder_agent.rb) to be able to download this attachment);
- Upload that attachment to dropbox – draft: PetroFeed@c947099;
- Detect changes on a given dropbox directory – this PR;
- Generate a temporary public link for a dropbox file – ToDo.

Cheers!
Yep, it does. And disables existing agents with a `Missing Gems` tag/message.