Child selectors is not work

In jsonselect not work first-child, nth-child, etc.
example:

```
from jsonselect import select

test_dict = {
    'a': 1,
    'b': 2,
    'c': [ 4, 5, 6, 7 ]
}


print select('.c :first-child',  test_dict)
print select('.c:first-child',   test_dict)
print select('.c :nth-child(2)', test_dict)
print select('.c:nth-child(2)',  test_dict)
```

returns:

```
None
None
None
None
```
