Performs some minor refactoring in Device API.
No changes to public API are made.

Adds Device.Write() method. This makes the Device type
implement the `io.Writer` interface.

Signed-off-by: Jim Teeuwen <jimteeuwen@gmail.com>
Adds the `testdata/morse` program. It illustrates a use-case
for the `ao` library in the form of a tool which translates
a commandline-supplied text string into an audible morse code signal.

Signed-off-by: Jim Teeuwen <jimteeuwen@gmail.com>