auto camelize keys

Hi

I've added support for keys autocamelizer when using serializers (many js frameworks have name convention for model fields so server should provide keys that way), but treat this as not as finished work and more as an idea. The best realisation from my perspective is to provide dsl like this:

```ruby
class SomeSerializer
  keys :camelize
```

or add opportunity to call any serializer method that will change keys
```ruby
keys :my_inflector
```

What do you think, will it be useful? If yes, then I'll add support for it.