Implement request group and pause/resume support

Ok, here's an initial take on the whole pause/resume support for Picasso.

This PR adds a new API to `RequestCreator` that allows you set a group tag/marker to specific requests:

```java
Picasso.with(context)
       .load("http://example.com/image.jpg")
       .group("mygroup")
       .into(someImageView)
```

By default, all requests are associated with the default group defined as `Picasso.DEFAULT_GROUP`. This tag can be used to cancel, pause, and resume requests in the same group. To do so, you can use the following new APIs in `Picasso`:

* `cancelRequestGroup(String group)`
* `pauseRequestGroup(String group)`
* `resumeRequestGroup(String group)`

Maybe we should also have `pauseRequest(Target|ImageView|RemoveView)` and `resumeRequest(Target|ImageView|RemoveView)` for parity with the existing `cancelRequest(Target|ImageView|RemoveView)`. Let me know what you think.

Here are a few general notes on how this new API behaves:

1. When you pause a group, any new request for the paused group will not be attached to any `BitmapHunter` until the group gets resumed. In this case, the requests for a paused group will be queued in the `Dispatcher` (see `pausedActions` member).
2. If a request group is paused while there are already on-going requests for it (i.e. `BitmapHunter` is either running or waiting for its turn), the requests in the now paused group will get detached from any pending `BitmapHunter` immediately. If the affected `BitmapHunter` ends up with no pending actions, it will get canceled (see `performPauseGroup`).
3. If a request is canceled while its group is paused, it will be removed from the queue of paused requests accordingly (see `performCancel`). This might happen, for example, if a new request is initiated for the same target than a paused request.
4. A `BitmapHunter` might contain `Actions` from multiple groups at any given time. Not likely to happen in practice, but it's something we have to handle. This is why there's no 1-to-1 mapping between `BitmapHunter` and requests groups.

I should probably add some more tests for the new APIs. But I wanted to get some early feedback before spending more time on them. I've added a super simple scroll listener based on the new API. Works like a charm ;-)
I think the idea of a default group is strange because its size changes based on how much or little you use the explicit groups. Is what we want the ability to control all groups (i.e., all requests) at once?