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 experimented with the webhooks, but found  it too cumbersome. I kept
receiving every single event for everything that happened (not only related
to file changes)... And the payload gives us user ids, we have to, then,
find those users and find out what changed using a "delta" api... So I
figured that a simple polling would be more than enough.

We can discuss it further if you need. I could've added this rationale to
the PR message itself.

On Sunday, October 12, 2014, Andrew Cantino <notifications@github.com>
wrote:

> In app/models/agents/dropbox_watch_agent.rb:
>
> > @@ -0,0 +1,131 @@
> > +module Agents
> > +  class DropboxWatchAgent < Agent
> > +    cannot_receive_events!
> > +    default_schedule "every_1m"
>
> This is probably okay, but I wonder if we should add webhooks and use the
> Dropbox Webhook API?
>
> —
> Reply to this email directly or view it on GitHub
> <https://github.com/cantino/huginn/pull/572/files#r18748742>.
>