Unused variable inspection

(Another attempt to merge - previous pull request #99 did not build. Rebased and trying again)

Closes #21

* better names of sample functions in tests
* smaller better named functions in UnusedVariableFinder
* added some additional support for variable defined in for clause of FLWOR expression
* basic handling of variable namespaces (by using prefix)

Not supporting global private variable yet - possible in another commit/another issue.