Bug fixes and UI improvements/corrections.

1. Bug fix Sigmoid Formula. And disable the Dynamic ISF when autosens.min = autosens.max or when autosens.min > 1 (safety) or when autosens.max < 1 (no use for DynISF in this case).

2. Make change of HbA1c unit persistent

3. Fix for delete ("-") button in Add Cabs View. Clears to "0" when deleted.

4. Statistics upload to NS fix. Add a default old Date value

5. UI fix for DecimalTextField. Display "" instead of 0 value when empty.

6. Readme updates.