Issue title: unused-namespaces linter warns if namespace vars only used inside syntax-quote forms
 Issue body: Example in the README docs for the unused-namespaces linter.

 Comments: 
 Comment 0: See also issue #25 

 Comment 1: There is a TBD in a comment before a test case in cases/testcases/unusednss.clj with an example.

 Comment 2: Should be fixed in Eastwood 0.2.5
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