Fixes for using-Refactorings

1. Extended 'Add Using' function by involving scope. New usings are
added to the inner (namespace-)scope, if it does have usings already.
2. Extended 'Remove unused import statements' function by removing and
sorting inside of scopes.
Merge pull request #2 from sven-n/fix-using-refactorings

Fixes for using-Refactorings