style: remove unused image resources

Relative to #81
style: change last update time preference label

Use shorter label: 'update time'
style(about): add logo under the app title
fix(station): update process throws android.database.sqlite.SQLiteConstraintException

Close #87
fix(update): the stations update always change the last update time in settings

Check if there are really new stations added

Close #88
chore: reactivate acra
chore: uses-sdk is always overriden by the gradle build script
chore: bump to 4.0.0
chore: remove legacy pom.xml
chore: exclude acra org.json dependency to remove build debug warnings
docs: update README

Change the year of the 1st release
Update libraries section
docs: update README

Add 4.0.0 release