Adds a more helpful installer.

This changes `rake alchemy:install` task:

* Redirects to `admin/pages` after signup, so the create language root form shows up directly after signing up
* Adds a `index` page layout (the new default for seeded language), that generates the new `article` element
* Adds an `article` element, that serves as example element with lots of hints and default texts
* Adds a post install message to point users to the `rake alchemy:install` task

This will help new Alchemy users to easier adopt.