rename extract() to extracting()

While `extract` is shorter, it doesn't read fluently, nor does it match the AssertJ api.

Therefore, rename this:

``` py
assert_that(people).extract('first_name').is_equal_to(['Fred','Bob'])
```

to this:

``` py
assert_that(people).extracting('first_name').is_equal_to(['Fred','Bob'])
```
