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.

My changes are released under BSD License (according to http://laputa.sharpdevelop.net/PatchSizedContributionsWithoutJCA.aspx I don't have to sign and send the JCA then). If this doesn't work this way anymore, let me know.