fill_value should default to 0 in add/sub/... numeric methods
I'm no scientist, but I think the intuitive behavior when adding or subtracting values is this: **if no explicit addition/subtraction/mathematical-operation is demanded and specified (on some values), do not change the values you already have**.

Instead, what pandas does is, with lack of explicit instructions, convert your nice data into useless NaNs. Here's an example:

``` python
>>> data = pd.Series(np.zeros(3))
>>> data
0    0
1    0
2    0
dtype: float64
>>> additional_data = pd.Series([3], index=[1])  # only partly defined
>>> data.add(additional_data)
0   NaN
1     3
2   NaN
dtype: float64
```

IMHO it would be much more intuitive to **assume no change** with **`fill_value=0` as default**.

``` python
>>> data.add(additional_data, fill_value=0)
0    0
1    3
2    0
dtype: float64
```

The philosophical shift is this: when the addend is omitted, instead of adding something that is not-a-number, we just don't add anything at all.

When one wants to botch their already-existing, nice data, they can always pass `fill_value=np.nan` explicitly. :-)

But I'm no scientist.
