Making DTSTART an optional while creating RRule

[sumitwy]

Google Calendar APIs don't accept DTSTART in RRule. Can we make it optional in the function
toRRuleString of RecurrenceRule. If you will agree to merge it into this repo, I can submit a pull request.

[tarangpatel]

@sumitwy can you show me how you did it ? I dont think this repo is not being updated.