Make it possible to set custom duplication method and also remap associations for created model

Use case
``` ruby
class ObjectPrototype < ActiveRecord::Base
  has_many :child_prototypes

  amoeba do
    method :become_real
    remapper :remap_associations
  end

  def become_real
    self.dup().becomes( RealObject )
  end
 
  def remap_associations( name )
    'childs' if name == 'child_prototypes'
  end
end

class RealObject < ObjectPrototype
  has_many :childs
end

class ChildPrototype < ActiveRecord::Base
  amoeba do
    method :become_child
  end

  def become_child
    self.dup().becomes( Child )
  end
end

class Child < ChildPrototype
end
```
Then:
``` ruby
prototype = ObjectPrototype.new
prototype.child_prototypes << ChildPrototype.new
object = prototype.amoeba_dup
object.class # => Object
object.childs.first.class #=> Child
```
It's on top of https://github.com/rocksolidwebdesign/amoeba/pull/21, so:
``` ruby
# class ObjectPrototype
...
def become_real
  if amoeba_dup_options[:build]
    self.dup().becomes( RealObject )
  else
    self.dup()
  end
end
...

prototype = ObjectPrototype.new
prototype.amoeba_dup.class  #=> ObjectPrototype
prototype.amoeba_dup({build:true}).class #=> RealObject
```