Warn when there is not used allowed rule

[leotsarev]

Given: There is some rule in nsdepcop. Say <Allowed From="*" To="Foo"> and there is no references to Foo namespace in csproj

Actual result: no warning Expected result: warning that rule is excess and could be removed.

Benefit: keeping your nsdepcop files nice and clean.

[realvizu]

Very good idea, thanks!