Allow for single dates in `*Post`s

Currently, many *Post instances use a date range, and assume that if the end date is absent, it's assumed that this date is Present (e.g., 2015-01-05 and None start and end dates are parsed as Jan 2015 — Present).

This precludes cases where an event lasted a short period of time — for example, a conference talk or poster are both dateable events, but typically are associated with just one date.