Return self from CollectionDecorator#replace

Closes #516
Fix generators path

Closes #526
Fix decoration of unsaved collection associations

Calling `#decorate` on an unsaved collection association forced a
db lookup, clearing any items that have been added (e.g. using
`#build`).

Closes #518.
delegate blank to source
Add spec for #blank? delegation
Fix build failure

rspec-rails 2.13.1 checks Rails.version when loading RailsExampleGroup
which is used by ammeter. Therefore we need to require rails.
use block_given in module_namespacing
Rename `source` to `object`

Closes #501
Rename `source_class` to `object_class`
Refactor name lookup