Summing with missing data appears to be broken in 0.16.2 when all elements are NA
Summing with missing data appear to be broken in 0.16.2.  According to http://pandas.pydata.org/pandas-docs/version/0.16.2/missing_data.html#calculations-with-missing-data
     When summing data, NA (missing) values will be treated as zero
     =>>> If the data are all NA, the result will be NA <== This returns zero in 0.16.2 (it returns NA in 0.15.2).
Old behavior as specified in documentation:
![pandas0 15 2feature](https://cloud.githubusercontent.com/assets/4975865/8936014/d61637ca-351d-11e5-8cb2-70bb28b2a28a.jpg)
New behavior which is different from documenation:
![pandas0 16 2bug](https://cloud.githubusercontent.com/assets/4975865/8936016/d7d6748a-351d-11e5-9518-2d3fcbc992be.jpg)
