fill_value kwarg for unstack
Currently:

``` python
In [2]: df = pd.DataFrame({'x':['a', 'a', 'b'], 'y':['j', 'k', 'j'], 'z':[0, 1, 2]})

In [3]: df.set_index(['x', 'y']).unstack()
Out[3]:
   z
y  j   k
x
a  0   1
b  2 NaN
```

If I want to fill with -1, i need to `fillna` and then `astype` back to `int`.  Ideally:

``` python
In [3]: df.set_index(['x', 'y']).unstack(fill_value=-1)
Out[3]:
   z
y  j   k
x
a  0   1
b  2  -1
```
