parse_date() and parse_iso_8601() now dupport a default time zone, that will be used for dates that do not explicitly specify one.
Reimplement parse_iso_8601() with vectorized code, for speed (#9).
Fix parse_date() and parse_iso_8601() for zero-length input (#20).
parse_date() parses strings with + characters correctly now (#23).
Drop lubridate package dependency
Fix parsing dates consisting of six or eight digits, e.g. 20140922 and 092214
NA is returned by parse_date for non-sensical numerical dates, e.g. 000202
Fix parse_date time zone that was wrong for some dates
Fix parse_date for dates are not in DST
format_iso_8601, on platforms that have a buggy %z