html.unescape is missing

Using future 0.15.2 in Python 2.7:

```
>>> from html import unescape
*** ImportError: cannot import name unescape
```

Using plain Python 3.4 it works.
The function was introduced in Python 3.4, see [documentation](https://docs.python.org/3/library/html.html#html.unescape).

Please add it.
