added a planning-comparison-service

The PlanningComparisonService restores tthe work_packages at a given time
from the latest journal-entries for the work_package. This commit
contains the sql to restore the work_packages and includes a simple
test to make sure, that the returned work_packages actually contain the
old attribute-values.
adding an additional journal-entry

this makes sure, that the journalization actually picks up the right
journal-entry.
Added the filters to planning-comparison
added the compare_date to the planning_elements_controller

The planning-elements-controller now accepts a historical date to
return work_packages at a given time.

There is now a cuke, that checks, that the api is actually returning
the historical values; there are also some general steps to use timecop
for producing historical data.
added the filtering to the cuke
replaced compare_date with at_time
fixing the current-workpackages: Historical workpackages can stil only be obtained for only one project
Now searching in multiple projects for historical data is possible
Added a cuke for the due_date-comparison; made the api more robust against missing filters.
work_packages are now resolved correctly when no filter is given
using DateTime instead of date to parse the give at_time