Remove Alchemy::Page's dependency to have the user extend ActiveModel
Rename Alchemy::Page model concerns to avoid namespace conflicts
Bump version to 3.0.0.rc6
Make DummyUser class inherit from ActiveRecord again.

This reverts changes from 7b1e66940ff744535469f0bbeb563adadfa68f13
Adds DummyUser factories.

The user factories where removed with extracting the user model into it's own gem. But is useful to have them for the dummy users.
Adds controller spec for locking page feature at page edit.
Do not raise not found error, if a page user could not be found. Fixes #560
Bump version to 3.0.0.rc7
Adds available_locales setter to Alchemy::I18n module.

With this feature developers can dictate which translations are provided for the user in the backend. (I.e. if Alchemy had 10 translations, but the developer wants to provide only 2 of them.)

It can be used with a simple initializer in the host app: `Alchemy::I18n.available_locales = [:en, :de]`
Refactors clipboard. Fixes #539

The clipboard was an abstract wrapper around Enumerable. Since Rails 4.1 only allows to store simple objects in the session, it was necessary to refactor the clipboard to be a simple Enumerable again.