Add Changelog
Add time duration constants
Fix and optimize time left calculation and display

Prior to this, there was a "gap" in display of sprint time left, causing
the progress bar to display no data if remaining time was >0 and <7200.

The "time left" display strings have been modified to use %d and sprintf
to facilitate i18n.

This commit uses the new duration constants instead of hardcoded values
to determine what to display.
Add plugin config page
Update README file
Bump version and update changelog