DatetimeIndex.insert works with only positive values
Consider allowing negative values for `loc` parameter of `DatetimeIndex.insert(loc, item)`.  Currently, negative parameters result in an "off by 1" return value.  For example:

```
In  [ 1]: index = pandas.DatetimeIndex(start = '01/01/2000', freq = 'B', periods = 5)
In  [ 2]: dt = pandas.Timestamp('01/01/2013')
In  [ 3]: new_ind = index.insert(-1, dt)
In  [ 4]: new_ind[-1]
Out[ 4]: Timestamp('2000-01-07 00:00:00', tz=None)
```

But:

```
In  [ 5]: new_ind[-2]
Out[ 5]: Timestamp('2013-01-01 00:00:00', tz=None)
```

I can put together a quick fork if it's a change you'd like.  If not, I think it makes sense to change the docstring to say that "negative values are not allowed."
