Feature/rails3 work package creation

This adds a work_package_controller. 

It's responsibility is to handle all work_packages. As long as issues and planning_elements are still separate classes, albeit inheriting from work_packages, the controller still differentiates between the two.

For now, the controller sports the following actions:

* show
* new
* create

Doing this required moving some of the functionality from the subclasses to work_packages itself:

* relations
* renames end_date to due_date

The original planning_elements- and issues-controller are left intact for now to allow a fallback when the work_package controller does not yet support what is required. On the other hand, links to those controllers are removed wherever possible. 

While creating the controller PermittedParameters needed to be adapted to allow for adding permitted parameters by plugins.