Fix for unused variables rules when handling namespace compact()

When `compact()` is called from a namespace AST parser returns fully qualified name of the function.
This means that `UnusedLocalVariable`, and `UnusedFormalParameter` rules fail because of this fact.

I tried to go through all the issues related to `compact()` and saw many patches. One of them (https://github.com/phpmd/phpmd/pull/61) apparently got somewhere lost.

This PR brings back checking only for `compact` in a fully qualified name.

I've created also a test suite to check if this works. I tried to keep convention of this particular tests - the ones related with `compact()` are named in snake case.  

Please review changes and let me know if they're ok.