ENH: name(s) argument for reset_index?
An idea: to be able to specify the a name when resetting the index, which would override the index name or the default value when index name is None.

Eg something like this:

```
In [10]: df = pd.DataFrame(np.random.rand(4,3), columns=list('ABC'), index=pd.date_range('2013-01-01', periods=4))
In [11]: df
Out[11]: 
                   A         B         C
2013-01-01  0.313664  0.606445  0.048081
2013-01-02  0.907785  0.004429  0.374456
2013-01-03  0.916584  0.067639  0.467712
2013-01-04  0.712824  0.687895  0.397960

In [12]: df.reset_index()
Out[12]: 
       index         A         B         C
0 2013-01-01  0.313664  0.606445  0.048081
1 2013-01-02  0.907785  0.004429  0.374456
2 2013-01-03  0.916584  0.067639  0.467712
3 2013-01-04  0.712824  0.687895  0.397960

In [12]: df.reset_index(name='datetime')
Out[12]: 
    datetime         A         B         C
0 2013-01-01  0.313664  0.606445  0.048081
1 2013-01-02  0.907785  0.004429  0.374456
2 2013-01-03  0.916584  0.067639  0.467712
3 2013-01-04  0.712824  0.687895  0.397960
```

For multi-index, you should provide a list. 
