ENH: Add default_index method to DataFrame
This partially addresses #1894. Just a method to reset the index in the sense of give me the default np.range index.

Looks like it does `df.reset_index(drop=True)`?

``` python
In [27]: df
Out[27]: 
          A         B         C         D
1  0.748854 -0.183158 -0.224337 -0.321686
4  1.000946 -1.383009 -0.950255  0.067827
6  0.445805 -0.994903 -0.152020 -1.445314
9 -0.900290  1.865405 -0.290509  0.138487

In [28]: df.reset_index(drop=True)
Out[28]: 
          A         B         C         D
0  0.748854 -0.183158 -0.224337 -0.321686
1  1.000946 -1.383009 -0.950255  0.067827
2  0.445805 -0.994903 -0.152020 -1.445314
3 -0.900290  1.865405 -0.290509  0.138487
```
 If so, then this should be clearer in the documentation for reset_index. 

```
drop : boolean, default False
    Do not try to insert index into dataframe columns
```

As I mentioned in the issue, I always assumed that reset_index should do what I want it to do, but I don't get that from the above.
