Add @unpack decorator

I propose to add decorator which will unpack argument:
```
    def process(foo, bar):
        ...
    
    map(unpack(process), product(['foo1', 'foo2'], ['bar1', 'bar2']))
```

This decorator already is in [ddt](http://ddt.readthedocs.io/en/latest/example.html) library, but it will be 
strange if I will install this module only for this.