Use transients instead of get_option

Replace get_option under twitter widget with get_transients. A better
and effective way to avoid conflicts between multiple use of the widget.
Change twitter widget transient default to 2 hours
Get Instagram video thumbnail for images
Remove unnecessary Instagram data
Fix a bug w/ Instagram image size
Delete plugin screenshots

Plugin screenshots are already moved to WordPress plugin repo’s assets/
directory
Tweak Instagram widget styles
Always use the Instagram lowercase username
Update changelog for 1.2.5
Version 1.2.5
Merge branch 'release-1.2.5'