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!
I'm not seeing how that would be possible with that gem. :disappointed:
In fact, `Dropbox::API::Config.app_key = consumer_key` and `Dropbox::API::Config.app_secret = consumer_secret` are only there because they are required by the gem, but they're only used if we were to use the gem's built-in oauth support. As we are not, those two entries should not be a problem, as far as I can see... Regarding `Dropbox::API::Config.mode = 'dropbox'` it means "look for stuff from the root of your dropbox accout", as opposed to `sandbox`, which is App/Folder specific...