w2utils.isTime() fixed & tweaked + unit tests added

cf. commit SHA-1: 89bd7a6c37fd36d8c19a77231b66efa30f31e6cb:

- w2utils.isTime() fixed & tweaked: it now rejects illegal time strings such as '001:000' and accepts the exception '24:00' as legal; it also now accepts AM/PM shorthand such as '3PM'.

- isTime came without unit tests. A series of pass/fail unit tests have been added.