add annotations

I've added a boatload of @Override and @SuppressWarnings("serial")
annotations. This commit shouldn't change how things work; it just gets
rid of warnings in Eclipse.
remove unused imports

Shouldn't change how anything works; just makes errors in Eclipse go
away.
remove unused variables

The Tester class (and the instance of it in the MainWindow) get an
@SuppressWarnings("unused") notation instead of removal.
an operator's name is accessed through toString()

Just because operator names are final doesn't mean we can't encapsulate
them!