Added error-handling to CSVHandler. CSVHandler throws an
UnsupportedOperationException with a fitting message that can be caught.
Added DAO<Dunning>
Added states and a method, closes issue#290
added class containing a message and a message type for a notification
system via Guava EventBus
example for posting a MessageEvent on an unsuccessful login attempt
added MessageEvent handler method in MainUI
 - shows a Vaadin Notification with the chosen style and message
Merge pull request #301 from davherrmann/issue#282

Issue#282
Replaced string comparison with == by string comparison with equals
Added parent header to csv.
Added error handling for the case that a student dataset does not
contain any parent information.
Inserted parent-testdata into csv.
Changed header name "Erz1StraßeNr" to "Erz1StrasseNr"
Added check for NumberFormatException to check if the postalcode is a
number. Changed postalcode in csv from "Test" to "1".
Changed propertyname from "Erz1StraßeNr" to "Erz1StrasseNr"
Merge remote-tracking branch 'upstream/develop' into issue#290
Merge pull request #303 from Junkern/issue#290

Issue#290
Added closeExistingWindow() to prevent an IllegalArgumentException
Merge remote-tracking branch 'upstream/develop' into issue#299
Merge pull request #306 from Junkern/issue#299

Issue#299
If no data is selected that could be printed the method
createByteArrayOutputStream of PDFHandler returns null now.
Added null checks to LendingView and ReturnView.
Replaced generic and deprecated code in PrintingComponent.
Merge pull request #302 from B3nRa/issue#184

Added error-handling to CSVHandler. CSVHandler throws an