[71412238] hide completed tasks after one day

Hide by default tasks that were completed 1 day ago or more. And show them if the user clicks on *Show completed tasks*
[pivotal story](https://www.pivotaltracker.com/n/projects/889184)
[71407728] fix dashboard heights

Hopefully will look better
[pivotal story](https://www.pivotaltracker.com/n/projects/889184)
[71407572] revise XLS headings

Do not mutate constants
[pivotal story](https://www.pivotaltracker.com/story/show/71407572)
Merge pull request #74 from bitzesty/71412238-hide-completed-tasks

[71412238] hide completed tasks after one day
Merge pull request #75 from bitzesty/71407728-fix-dashboard-heights

[71407728] fix dashboard heights
Readme update
Merge pull request #76 from bitzesty/71407572-revise-xls-headings

[71407572] revise XLS headings