Option to warn when dropping NAs
In pandas 0.22.0, 

```
import pandas as pd
import numpy as np
pd.Series([1.0, np.nan]).sum()
```
gives

```
1.0
```

This is dangerous behavior in data analysis. Users often do want to drop NA values, and having to specify `dropna=True` manually may be annoying, but dropping NAs is fraught. 

Could this be an option -- perhaps  `pandas.set_option('warn_drop_na', True)`? The user may drop NAs consciously beforehand, or explicitly during the operation, if they want this behavior without a warning, when this option is enabled.

For comparison:

In R,

```
> sum(c(1.0, NA))
[1] NA
> sum(c(1.0, NA), na.rm=TRUE)
[1] 1
```

The SQL specification says,

> If one or more null values are eliminated, then a completion condition is raised: warning- null value eliminated in set function.

