add preview image for widget
upgrade tools, downgrade okhttp
show error message in widget if necessary
add more click listener for widget
Merge branch 'master' into widget_preview

Conflicts:
	app/src/main/res/values/strings.xml
	gradle/wrapper/gradle-wrapper.properties
open event in GDG app if no g+ link available in widget
Merge pull request #150 from gdg-x/widget_preview

add preview image for widget