Added app.close() method
Added app.close() method to gracefully kill servers started with app.listen(). Includes a unit test.

Fixes https://github.com/senchalabs/connect/issues/967

Note: the test "connect.static() on ENAMETOOLONG should next():" was failing on my system before and after this change, however the number of passing tests increased from 226 to 227 so I'm confident that my change works.