passing index parameter shouldn't be necessary when applying pivot function
Hello,

I have a Pandas DataFrame `df` like this:

```
            name  value
2014-01-01  temp   10.0
2014-01-01     p  101.0
2014-01-02  temp   11.5
2014-01-02     p  100.0
```

I wanted to get a new DataFrame `df2`like:

```
            temp      p
2014-01-01  10.0  101.0
2014-01-02  11.5  100.0
```

I did this:

```
df2 = df.pivot(columns='name', values='value')
```

but it raises `cannot label index with a null key`

I need to do this

```
df2 = df.pivot(index=df.index, columns='name', values='value')
```

see http://stackoverflow.com/questions/26101246/spread-data-across-multiple-columns

In my mind, passing index parameter shouldn't be necessary when applying pivot function in such a case.

Kind regards
