Pandas Interpolate Does not Handle Empty Values at Front of Series
[This code](https://github.com/pydata/pandas/blob/7d6fb510c1dfdbe7342f32f05ca5fd69b7854081/pandas/core/common.py#L1646) means the interpolator won't handle extrapolating to the front of the Series even though the underlying implementations may have no problem with the extrapolation. 

See for example [`UnivariateSpline`](http://docs.scipy.org/doc/scipy-0.15.1/reference/generated/scipy.interpolate.UnivariateSpline.html) whose default behavior is extrapolation.

Interpolation works fine at the end of the Series.

For example:

``` python
s = pd.Series([1, 2, 3, 4, np.nan, 6, np.nan])
s.interpolate(method='spline', order=1)
0    1
1    2
2    3
3    4
4    5
5    6
6    7
dtype: float64
```

but:

``` python
s = pd.Series([np.nan, 2, 3, 4, np.nan, 6, 7])
s.interpolate(method='spline', order=1)
0   NaN
1     2
2     3
3     4
4     5
5     6
6     7
dtype: float64
```
