Add CustomFieldOptions.Id property

Dear Eric,
This is my first 'pull request' in GitHub ever, so this might not be the good way to proceed.
Feel free to let me know.

I would like to suggest this patch, which adds an 'id' field in the CustomFieldOptions.
This would allows updating the CustomFieldOptions of a TicketField.
I have included a UnitTest to demonstrate the use case which would be covered.

Regards,
Gilles