Remove ToggleDrawable, check nulls, add transitions in elements
Merge remote-tracking branch 'origin/master'

Conflicts:
	app/src/main/java/es/openkratio/colibribook/MainActivity.java
	app/src/main/res/layout/activity_main.xml
	app/src/main/res/values-v11/styles.xml
	app/src/main/res/values-v21/styles.xml
	app/src/main/res/values/styles.xml
Update Gradle wrapper and dependencies, fix some warnings