New formats added for 'week()' function and 'default_week_format' option(4 - 7).
Next formats is supported now:
*Value* *Meaning*
  `0'     Week starts on Sunday; First Sunday of the year starts week 1.
  	  Week() returns 0-53.
  `1'     Week starts on Monday; Weeks numbered according to ISO 8601:1988.
	  Week() returns 0-53.
  `2'     Week starts on Sunday; First Sunday of the year starts week 1.
  	  Week() returns 1-53.
  `3'     Week starts on Monday; Weeks numbered according to ISO 8601:1988.
	  Week() returns 1-53.
  `4'     Week starts on Sunday; Weeks numbered according to ISO 8601:1988.
	  Week() returns 0-53.
  `5'     Week starts on Monday;  First Monday of the year starts week 1.
  	  Week() returns 0-53.
  `6'     Week starts on Sunday; Weeks numbered according to ISO 8601:1988.
	  Week() returns 1-53.
  `7'     Week starts on Monday;  First Monday of the year starts week 1.
  	  Week() returns 1-53.