BUG: pivot_table may raise TypeError without values
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry

`pivot_table` raises `TypeError` when `index` or `columns` is array-like and `values` is not specified.

```
df = pd.DataFrame({'A': [1, 2, 3, 4, 5]},
                  index=pd.DatetimeIndex(['2011-01-01', '2011-02-01', '2011-01-02', '2011-01-01', '2011-01-02']))
df.pivot_table(index=df.index.month, columns=df.index.day)
# TypeError: unhashable type: 'numpy.ndarray'
```
