DataFrame.head(0) doesn't return an empty frame
``` python
pandas.DataFrame({"stuff":[1,2,3,4,5]}).head(0)
```

returns

``` python
   stuff
0      1
1      2
2      3
3      4
4      5

```

I would expect an empty data frame, in the same way that

``` python
[1,2,3,4,5][0:0]
```

returns an empty list.

This is with pandas version 0.15.0.

The documentation (http://pandas.pydata.org/pandas-docs/version/0.17.0/generated/pandas.DataFrame.head.html) says `head(n)` returns 'first n rows', so I'd expect 0 rows if n is 0.
