Feature/individual entity

I'm sure there is a nicer way to do this, but hopefully this gets oat moving in the right direction. 
Adds support for individual resources at the top-level in JsonAPI without breaking existing behaviors.

The spec says:
> An individual resource SHOULD be represented as a single "resource object" (described below) or a string value containing its ID (also described below).

This change works by setting a flag in the serializer ('individual') that indicates you want a singular resource. 

I originally modified the inflection rules as well (so if you specify 'individual' we don't try to pluralize the names), but then I decided against it and went back to pluralizing everywhere for consistency. You can see that moment of self-doubt in the commit history.

Note: it might be useful to _also_ include a flag to turn off pluralization for those few cases where a plural name makes no sense (like in this case, where I was writing /api/status)

Usage:
```
class Status < Oat::Serializer
  adapter Oat::Adapters::JsonAPI
  schema do
    individual
    type 'status'
    map_properties *%i(at status)
  end
end
```

Output:
```
{
    "statuses": {
        "at": "2014-09-21T08:24:12.807-07:00",
        "status": "unknown"
    }
}
```