Merge remote-tracking branch 'upstream/rails3' into twitter-bootstrap

Conflicts:
	Gemfile
Outlined new concept for multiple periodically recurring tasks (#3).
Merge commit '1cdb9e85017a17e4dbee276cd87feaa2aab75932' into rails3-multiple-recurring-tasks

Conflicts:
	Gemfile
	db/schema.rb
Merge remote-tracking branch 'upstream/master' into multiple-recurring-tasks

Conflicts:
	config/locales/de.yml
Fixed schema.rb.
Users may delete all tasks of a group, i.e. delete the whole group.
Now creating periodic task via button
Exclude tasks from groups when changing attributes.
Delete only the FOLLOWING tasks.
Removed unused I18n strings.
Added up and down migration for weekly task data.
Removed obsolete code of old workgroup-centric weekly task concept.
Add validation for task.done in combination with periodic task.
Validate done/weekly combination only on creation of task.
Skip save callback when migrating data.
Remove obsolete columns from database.
Added i18n strings.
Added missing migration files.
Add repeated symbol to task list.
Added warning/ hint when updating periodic task.