Added FolderChildrenTests and FolderTests

* implemented some testcases
* fixed some issues

main/filesystem-api/src/main/java/org/cryptomator/filesystem/Deleter.java

@@ -0,0 +1,15 @@
+package org.cryptomator.filesystem;
+
+public class Deleter {
+
+	/**
+	 * Deletes all and only the content of a given {@link Folder} but <b>not</b> the folder itself.
+	 */
+	public static void deleteContent(Folder folder) {
+		if (folder.exists()) {
+			folder.folders().forEach(Folder::delete);
+			folder.files().forEach(File::delete);
+		}
+	}
+
+}

main/filesystem-api/src/main/java/org/cryptomator/filesystem/File.java

@@ -75,4 +75,10 @@ public interface File extends Node, Comparable<File> {
 		Mover.move(this, destination);
 	}
 
+	default void delete() {
+		try (WritableFile writableFile = openWritable()) {
+			writableFile.delete();
+		}
+	}
+
 }

main/filesystem-api/src/main/java/org/cryptomator/filesystem/Folder.java

@@ -31,7 +31,7 @@ public interface Folder extends Node {
 	 * @return the created {@code Stream}
 	 * @throws UncheckedIOException
 	 *             if an {@link IOException} occurs while initializing the
-	 *             stream
+	 *             stream or the {@code Folder} does not exist
 	 */
 	Stream<? extends Node> children() throws UncheckedIOException;