Accepting not-yet-issued propeties in a crowdsale

[dexX7]

This seems currently forbidden by the spec. Is there another reason besides "accepting an invalid property makes no sense?" If none, I suggest to remove this condition, because:

1. It adds an overhead (additional validation check) which adds no value
2. Remove obstacles that might interfere with scheduled / systematic issuance of properties that is somehow combined with a new crowdsale.
3. Factor x - the unknown. The probably very stupid idea I had first in mind: there could be a game where some tokens are issued "for free" for the "winner" that "gets" the property identifier. ;)