Add DateSet, bug #433704.

Add DateSet, bug #433704.

Allow sets of packages installed before or after a specific date.
The date may be specified explicitly (in dateformat which defaults to "%x %X")
or seconds since Epoch, or implicitly as an installation date or a filestamp.

For example, the following sets can be specified in /etc/portage/sets.conf:

[date1]
class = portage.sets.dbapi.DateSet
mode = older
date = 08/31/12 10:00:00
dateformat = %%x %%X

[date2]
class = portage.sets.dbapi.DateSet
mode = newer
seconds = 1346400000

[date3]
class = portage.sets.dbapi.DateSet
package = sys-devel/gcc:4.7

[date4]
class = portage.sets.dbapi.DateSet
filestamp = /usr/bin/gcc