Resolve "Java monades code"

Closes #111 (closed)

java-monades/src/main/java/fr/ippon/monad/Maybe.java

+package fr.ippon.monad;
+
+import java.util.Optional;
+import java.util.function.Function;
+
+public class Maybe<T> {
+
+  private final Optional<T> value;
+
+  private Maybe(Optional<T> value) {
+    this.value = value;
+  }
+
+  public static <T> Maybe<T> pure(T value) {
+    return new Maybe<>(Optional.of(value));
+  }
+
+  public static <T> Maybe<T> fail() {
+    return new Maybe<>(Optional.empty());
+  }
+
+  public <U> Maybe<U> map(Function<T, U> mapper) {
+    return new Maybe<>(value.map(mapper));
+  }
+
+  public <U> Maybe<U> bind(Function<T, Maybe<U>> other) {
+    return value.map(other)
+        .orElse(Maybe.fail());
+  }
+
+  @Override
+  public String toString() {
+    return value.toString();
+  }
+}