Add __getslice__ method to newbytes

Currently in Python 2.x:

```
>>> from future.builtins import bytes
>>> b = bytes(b'123')
>>> type(b[:])
<type 'str'>
```
Should be:
```
>>> from future.builtins import bytes
>>> b = bytes(b'123')
>>> type(b[:])
<class 'future.builtins.backports.newbytes.newbytes'>
```

For some reason slicing calls not \_\_getitem\_\_ with slice object as argument but \_\_getslice\_\_ although it is [deprecated since 2.0](http://docs.python.org/2/reference/datamodel.html#object.__getslice__).