Add Windows support

Set the default serial port for Windows to COM3.
Pass scons.bat (rather than scons) to subprocess.Popen on Windows.
Run scons.bat from the directory in which it resides (by passing
subprocess.Popen: cwd=<python_installation_dir>/Scripts).
Kill child processes portably use Popen.terminate() rather than
os.kill().
Prevent 'Access Denied' errors on Windows due to the parent
process holding open the serial port whilst a child process
(scons/avrdude) tries to open it. This is fixed by opening the
serial port after the test is uploaded, retrieving the test results
and closing the serial port again prior to the next upload.

This fixes #5.