Implement batch actions.

This change implements batch actions on collections:

```python
s3.Bucket('boto3').objects.delete()
```

It does the following:

* Create a `CollectionFactory` class to generate subclasses of
  `CollectionManager` and `ResourceCollection`.
* Update the `ResourceFactory` to use the new `CollectionFactory`.
* Add a public `pages()` method to collections. This returns entire
  pages of resource instances and it used by `__iter__` as well.
* Add batch actions as methods via the new `CollectionFactory`.
* Add a `BatchAction` subclass of `Action` which does the following:

    1. Get a page of results from the collection's operation
    2. Build parameters for the batch action operation
    3. Call the batch action operation
    4. Repeat until no more pages

* Makes some previously public members private on `Action` as these
  should never have been public.
* Update documentation to include collection classes.
* Add tests to cover the new functionality.
Merge pull request #32 from boto/batch-actions

Implement batch actions.