Renames Tracker into Type.
Fixes old migrations w/ trackers.

This adds the old tracker model to the migrations that use it. The code
for the migrations was not changed, as the const name is set to Tracker
via

  Object.const_set("Tracker", AddTrackerPosition::Tracker)

Each migration removes the Tracker constant afterwards with

  Object.send(:remove_const, :Tracker)
Adds migration that renames trackers to types.
Moves some issue specs to work_package specs.
Renames Tracker to Type in lib, etc. folder

This includes data loader, pdf export, email and reminder Rakefiles, as
well as a few random places that I missed before.
Updates routes from Tracker to Type.
Changes translations of Tracker to Type.
Changes Tracker to Type in features.
Changes Tracker to Type in tests.
Changes Tracker to Type in fixtures.
Fixes two cukes.

Type is used for STI, not for getting the type.
Merge branch 'feature/rails3' into feature/rename_tracker_to_type

Conflicts:
	app/views/journals/diff.html.erb
Fixes a step name.
Renames STI column name on work packages.
Fixes url generation work with new sti column name.

Type would otherwise collide with the Type model.
Fixes fixtures after rename of sti column.
Fixes URL for planning element generation.
Fixes hidden field in work package new action.

This STI type field was rather elusive.
Fixes more controller specs.
Merge remote-tracking branch 'opf/feature/rails3' into feature/rename_tracker_to_type