Re-use the serial monitor Terminal window

* ArduinoSerialMonitor no longer deploys the code, it simply launches
 the serial monitor. This allows you to launch the monitor after
 deploying without re-deploying. It also fixes a recursion bug if you
 have auto_open_serial set.
* The screen session is named. This allows us to kill the session before
 each deploy, allowing you to deploy without manually stopping the
 serial monitor
* The AppleScript uses the native Terminal application, removing the
 iTerm dependency.
* The AppleScript uses the same Terminal window for serial monitoring.
 If a serial monitor tab doesn't exist it creates one.

 Basically, this commit allows you to open the serial monitor at will,
 and decouples serial monitor management from deployment. You can
 <leader>ad at will and the monitor will always be up to date.