Update scrollbar functionality of .to-do

Ensure the scrollbar of the .to-do list is selectable for  Windows users. An explicit z-index was needed to ensure the scrollbar for the element using .to-do list was in fact selectable.
Merge pull request #1 from lozandier/patch-1

Fixes overflow issue on .to-do list scrollbars
Fixing scroll and z-index issues found by Kevin for Windows Chromers.