Dynamic attributes

By default, Couchbase-orm requires all attributes that may be set on a document to be explicitly defined using attribute declarations. Couchbase-orm should also supports creating attributes on the fly from an arbitrary hash or documents stored in the database.

```ruby 
class Person < CouchbaseOrm::Base
 
end

bob = Person.new(name: 'Bob', age: 42)
bob.name
# => "Bob"
```

