ENH: Index optional pivot
Closes #3962.

Addressed comments.
 Failures look unrelated?
 lgtm. can you add a release note refing the original issue. squash, then ping when green.
 Done
 - can you update the doc-strings of both `pd.pivot` and `DataFrame.pivot` (saying `index` is optional/can be `None`).
- can you add a test using `pd.pivot`?
 Are there any tests for `pd.pivot`? It's just a pass-through.

**Edit** Ah, you mean `pivot_simple`, which is different than `pivot` but imported into the namespace as `pivot`? Yeah, I didn't touch this code. I suppose we should just create a default index in this case?
 looks `pd.pivot_table == DataFrame.pivot_table`. While `pd.pivot == DataFrame.pivot`.

So ok on `.pivot`. Maybe have a look at `.pivot_table`?
 It appears to work.

```
In [8]: df
Out[8]:
     A    B      C  D
0  foo  one  small  1
1  foo  one  large  2
2  foo  one  large  2
3  foo  two  small  3
4  foo  two  small  3
5  bar  one  large  4
6  bar  one  small  5
7  bar  two  small  6
8  bar  two  large  7

In [9]: pd.pivot_table(df, values='D', index=None, columns=['C'], aggfunc=np.sum)
Out[9]:
C
large    15
small    18
Name: D, dtype: int64

In [10]: pd.pivot_table(df, values='D', index=['A', 'B'], columns=['C'], aggfunc=np.sum)
Out[10]:
C        large  small
A   B
bar one      4      5
    two      7      6
foo one      4      1
    two    NaN      6
```
 ok gr8. test pretty pretty pls.
 It's tested. https://github.com/pydata/pandas/blob/master/pandas/tools/tests/test_pivot.py#L60
 ic, I guess that already accepted `None` then, ok.
 ok, just fix the doc-string and push again.
 Done.
 ok cool. ping when green.
 thank you sir!
