Adds configurable default adapter

This patch adds the ability to configure a default adapter on
ActiveModel::Serializer as described in the README:

```ruby
ActiveModel::Serializer.default_adapter = ActiveModel::Serializer::Adapter::HalAdapter
```

The current default adapter, when no other default adapter is specified
(or when the default adapter is set to nil), is set to the NullAdapter
because that's all that we have at the moment to work with. Future work
should change the out-of-the-box default adapter to the `JsonApiAdapter`
when we have this adapter ready.
Use configured adapter when rendering JSON

Update our JSON renderer to use the configured `default_adapter` when
rendering JSON. This patch also updates the ActionController
serialization test to explicitly use the NullAdapter so that we can be
robust against changes to whatever our out-of-the-box `default_adapter`
may be.
Remove some extra blank lines

Snip snip!
Merge pull request #1 from andrewle/configurable_default_adapter

Configurable default adapter