Feature/rails3 remove planning element

This sums up quite a lot of changes that were required to remove the planning_element and get the specs&cukes to run again. 
  
  * deleted PlanningElement and the corresponding factory
  * mechanical replacements of planning_element by work_package
  * fixes in the moves_controller to get a proper handing of permitted-params
  * feature steps renamed where possible
  * several fixes to the api/v2/planning_element_controller to make sure, that everything is looked up in .work_packages instead of .planning_elements
  
There are still some references to the planning_elements left that we should erase over time, and we need to do some manual testing(e.g. is PlanningElementColor still handled correctly?!).