Update palm_soc.py

[grahamhobson]

Changes include:

1. New command line argument (-o) added to enable once mode (immediate execution then exit). This is useful if the application is run from a cron job or iOS scheduled event.
2. When using once mode, setting inverter start_time and end_time is now optional. They can be set to empty strings in settings.py and the times won't be updated in the inverter registers (for users that want to control times through the GivEnergy app).
3. Added timestamp to start of main loop logging. Useful for log analysis.
4. Corrected one typo in log narrative.