Allow custom class name in has_many relationship

# Why?
Say you have a model that can add child models of the same type in a hierarchy. If they're all the same type, Arkenstone couldn't work with it.

# What Changed?
Now you can specify a `class_name` on a `has_many` relationship.

```ruby
class Organization
  url 'http://example.com/organizations'
  attributes :name
  has_many :descendents, class_name: :organization
end

org = Organization.find 100
org.descendents
# spits out an array of Organizations from 
# http://example.com/organizations/100/descendents
```