Fixed week numbers in date picker

Changed week number definition to ISO 8601 which is the following:
"weeks start on a Monday; the first week of the year contains the first
Thursday of the year".
Moved ID tags to the upper left corner

Instead of having ID tags in the bottom right corner of each backlog
item, it is now placed in the upper left corner together with the label
that indicates whether the item is a task, story, epic or theme.
Updated link for screenshots in README

The link became broken when sonyericssondev changed name to
sonyxperiadev.
Merge pull request #34 from fredrikpersson/id-tag

Moved ID tags to the upper left corner
Merge pull request #35 from fredrikpersson/readmelinkfix

Updated link for screenshots in README
Merge pull request #33 from fredrikpersson/weekfix

Fixed week numbers in date picker