Changes to filesystem API and nio implementation

* Partial implementation of nio filesystem
* Addded some tests
* Added project for common test dependencies
* Removed default implementation of Folder#delete
** reason: didn't work because empty folders were not deleted and this
cannot be done in the default implementation

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

@@ -0,0 +1,86 @@
+package org.cryptomator.filesystem;
+
+import java.util.function.Consumer;
+
+public class FileSystemVisitor {
+
+	private final Consumer<Folder> beforeFolderVisitor;
+	private final Consumer<Folder> afterFolderVisitor;
+	private final Consumer<File> fileVisitor;
+
+	private FileSystemVisitor(Consumer<Folder> beforeFolderVisitor, Consumer<Folder> afterFolderVisitor, Consumer<File> fileVisitor) {
+		this.beforeFolderVisitor = beforeFolderVisitor;
+		this.afterFolderVisitor = afterFolderVisitor;
+		this.fileVisitor = fileVisitor;
+	}
+
+	public static FileSystemVisitorBuilder fileSystemVisitor() {
+		return new FileSystemVisitorBuilder();
+	}
+
+	public FileSystemVisitor visit(Folder folder) {
+		beforeFolderVisitor.accept(folder);
+		folder.folders().forEach(this::visit);
+		folder.files().forEach(this::visit);
+		afterFolderVisitor.accept(folder);
+		return this;
+	}
+
+	public FileSystemVisitor visit(File file) {
+		fileVisitor.accept(file);
+		return this;
+	}
+
+	public static class FileSystemVisitorBuilder {
+
+		private Consumer<Folder> beforeFolderVisitor = noOp();
+		private Consumer<Folder> afterFolderVisitor = noOp();
+		private Consumer<File> fileVisitor = noOp();
+
+		private FileSystemVisitorBuilder() {
+		}
+
+		public FileSystemVisitorBuilder beforeFolder(Consumer<Folder> beforeFolderVisitor) {
+			if (beforeFolderVisitor == null) {
+				throw new IllegalArgumentException("Vistior may not be null");
+			}
+			this.beforeFolderVisitor = beforeFolderVisitor;
+			return this;
+		}
+
+		public FileSystemVisitorBuilder afterFolder(Consumer<Folder> afterFolderVisitor) {
+			if (afterFolderVisitor == null) {
+				throw new IllegalArgumentException("Vistior may not be null");
+			}
+			this.afterFolderVisitor = afterFolderVisitor;
+			return this;
+		}
+
+		public FileSystemVisitorBuilder forEachFile(Consumer<File> fileVisitor) {
+			if (fileVisitor == null) {
+				throw new IllegalArgumentException("Vistior may not be null");
+			}
+			this.fileVisitor = fileVisitor;
+			return this;
+		}
+
+		public FileSystemVisitor visit(Folder folder) {
+			return build().visit(folder);
+		}
+
+		public FileSystemVisitor visit(File file) {
+			return build().visit(file);
+		}
+
+		public FileSystemVisitor build() {
+			return new FileSystemVisitor(beforeFolderVisitor, afterFolderVisitor, fileVisitor);
+		}
+
+		private static <T> Consumer<T> noOp() {
+			return ignoredParameter -> {
+			};
+		}
+
+	}
+
+}

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

@@ -88,17 +88,7 @@ public interface Folder extends Node {
 	 * <p>
 	 * If the directory does not exist this method does nothing.
 	 */
-	default void delete() throws UncheckedIOException {
-		if (!exists()) {
-			return;
-		}
-		folders().forEach(Folder::delete);
-		files().forEach(file -> {
-			try (WritableFile writableFile = file.openWritable()) {
-				writableFile.delete();
-			}
-		});
-	}
+	void delete();
 
 	/**
 	 * Moves this directory and its contents to the given destination. If the