Consider how strict to be in verifying URNs

[sedwards@bbn.com]

#113 made a change to gmoc.py to be tolerant of # in URNs even though # isn't a legal character.

We should consider how tolerant we want to be in general for both methods listed below.

In particular, when isValidURN() is used directly then it's always used to create a URN if a string is passed in. For example, this is used to convert a string into a URN (if it isn't already one) for it to be the ID of a Sliver.

However validateURN() calls isValidURN() and is used to verify that a URN follows the rules. For example, this is used to make sure Resources and Slices have valid URNs for their IDs.