Added a sample project to the development-seed.[skip ci]

using RAILS_ENV=development rake db:seed, you can now generates a sample
project: The project contains several issues, a timeline and lots of
planning-elements to play with. Feel free to add some more sample data
to this pet-project.

The project is re-generated every time you seed the database, so every
changes to this project will be lost.

The ci is currently skipped, as it currently does not run the seeding.
Added some fixes for the seeding, fixed the remaining gem-dependencies[skip ci]
fixed a misleading comment.
Merge pull request #339 from opf/feature/rails3_development_seed_project

Add a sample project to the development-seed.