Default date limits changed to 1970-01-01 to 1970-01-02#

Previously the default limits for an empty axis set up for dates (Axis.axis_date) was 2000-01-01 to 2010-01-01. This has been changed to 1970-01-01 to 1970-01-02. With the default epoch, this makes the numeric limit for date axes the same as for other axes (0.0-1.0), and users are less likely to set a locator with far too many ticks.