Finish issue #11 (?) with this first crack at heirarchical tag support.
Finish issue #11 (?) with this first crack at heirarchical tag support.

* lib/tk_main.py
  (TKEntryKey._compare_tags): New.
  (TKEntryKey.__cmp__): Use _compare_tags for more precise sorting.
  (TKEventTagTree.GetTagStack): Parse tags as '/'-delimited segments, and
    now only return the stack items that exist (no padding with None
    values).
  (TKEventTagTree.EntryChangedListener): Parse tags as '/'-delimited
    segments, updating the call to GetTagStack() and the handling of
    its variable-length returned list.
  (TKEventTagTree.OnCompareItems): Lose as unnecessary.

Isendir/thotkeeper,cmpilato/thotkeeper