Add a Beep() method to the Screen interface. On *nix systems, this
writes the bell character (0x07) to the tty. On Windows, we call the
MessageBeep syscall.
Fixes: #2
While go-convey was pretty nice, it carries a rather large dependency
graph, which we think it is better not to burden our downstreams with.
It is easy enough to just refactor the tests to use the standard testing
package.
