- then feature for BddMockito

New feature in BddMockito for BDD then part of the test which allows mock behavior verification by using `Mockito#verify(Object)`.
For example we have classes Person and Bike:

```java
Bike bike = new Bike();
Person person = mock(Person.class);
```

and the following interaction

```java
person.ride(bike);
person.ride(bike);
```

Current (non BDD) behavior verification would go like:

```java
verify(person, times(2)).ride(bike);
```
This feature introduces the following way of verification:

```java
then(person).should(times(2)).ride(bike);
```