Default date renderer now handles "zero" dates (0000-00-00...)

The default date format renderer now checks for the case of a "zero"
date ('0000-00-00' or '0000-00-00 00:00:00') and returns it as-is (but
in grey font) instead of passing it through a formatter (since the
default formatter pitifully returns things like 'Nov 30, 00-1 12:00 AM')

Just in case the user doesn't want this for some reason, there is a second
optional argument bypass flag (see code)

Also generalized new css class 'ra-null-val' which sets color:darkgrey
and is now used by the standard Ext.ux.showNull renderer as well as the
new zero date in the date renderer (instead of a hard-coded style)