Support AssertJ

[azplanlos]

This PR enables the usage of AssertJ next to Hamcrest.

There are several options to perform assertions on a thread:

Fluent usage with SoftAssertions:

final Waiter w = new Waiter();
new Thread(new Runnable() {
  public void run() {
    w.softlyAssertThat("one").isEqualTo("two");
    w.assertAndResume();
  }
}).start();
w.await(1000);

Mutiple soft assertions:

final Waiter w = new Waiter();
new Thread(new Runnable() {
  public void run() {
    SoftAssertions assertions = new SoftAssertions();
    assertions.assertThat("one").isEqualTo("two");
    w.assertThat(assertions);
  }
}).start();
w.await(1000);

Using a consumer:

final Waiter w = new Waiter();
new Thread(new Runnable() {
  public void run() {
    w.assertThat("one", (assertion) -> assertion.isEqualTo("two"));
  }
}).start();
w.await(1000);

Using a lambda function with static imports:

final Waiter w = new Waiter();
new Thread(new Runnable() {
  public void run() {
    w.assertThat(() -> assertThat("one").isEqualTo("two"));
  }
}).start();
w.await(1000);