Add Facebook Sharing, Delete Pictures

This update is not major, but I did change a bit internally. The biggest changes in v1.3.1 are the ability to share on Facebook and the sharing menu is now a dropdown menu.

Changelog

New

• Added APOD Twitter link to popup.html footer
• Added sharing on Facebook support

Changed

• Changed the way the Twitter sharing function works
• Changed sharing from popup to dropdown menu

Fixed

• Fixed sharing not displaying when image date entered manually
• Removed pictures that were not in use