Can Transaction.date be optional?

[gordonje]

In California, campaign expenditures are reported on Schedule E of Form 460 which does not provide an obvious place for filers to report the date of a payment made. Thus, about 40% of Form 460 Schedule E Items are missing an expense date.

In the current version of the draft spec Transaction.date isn't labeled as optional, but maybe it should be?

In the draft implementation in python-opencivicdata, I'm going to allow NULLs in this field, for now.

This sort of overlaps with #98 in terms of how these decisions facilitate longitudinal analysis of transaction data.

[aepton]

Added to #104