Added the scaffolding for the burn down chart.

Created a new class: Burndown. Extended the Sprint class so that it gets all changes and associated dates from the Podio API. The Burndown instance is stored within a Sprint instance.
Removed unnecessary echo’s and var_dump’s
Changed indexing of the change array: from timestamps to days
Added the function to export the burn down chart data in the Google Charts format
Added the Google Chart JS dependencies and the burn down chart
Centered the chart on the page
Improved the way data is displayed in the chart and added indication of the current day
Fixed the current day calculation