Add assertThat(new ExceptionalRunnable {...}).toThrow(Exception.class) API

This commit makes it possible to write things like this:

```java
    assertThat((ExceptionalRunnable) () -> { 
        throw new IllegalArgumentException(); 
    }).toThrow(IllegalArgumentException.class);
```

of on Java 7:

```java
    assertThat(new ExceptionalRunnable() {
      @Override
      public void run() throws Exception {
        throw new IllegalArgumentException();
      }
    }).toThrow(IllegalArgumentException.class);
```