Import of a tracker fails with metakit backend.

Enter tracker home: .
Traceback (most recent call last):
  File
"/home/kent/src/roundup/work/roundup/roundup/admin.py",
line 1287, in run_command
    ret = function(args[1:])
  File
"/home/kent/src/roundup/work/roundup/roundup/admin.py",
line 1128, in do_import
    cl.import_journals(reader)
  File
"/home/kent/src/roundup/work/roundup/roundup/backends/back_metakit.py",
line 1720, in import_journals
    elif isinstance(prop, Date):
NameError: global name 'Date' is not defined

And there are same errors for Interval and Password fields.

Patch included to fix this.