Strip extra whitespace

Clean up the file by removing extraneous whitespace.
Use is_ready() check instead of connected check

Make send_bytes and receive_bytes check self.is_ready() instead of
self.connected, which means something different in Calculator than it
did in Connection.
Add send_key method

Add a method for sending keys to a calculator. This does not work in
testing using a TI-84+ SE over DirectLink on Linux, but it is a ticalcs
issue that I don't know how to fix. The same non-working behavior was
exhibited in a test C program.