sum over objects
Was surprised by this. I don't recall seeing this behavior before. I guess there are some object types that you do want to try to add together? Still seems like this could lead to problems

```
[~/statsmodels/statsmodels-skipper/statsmodels/sandbox/examples/]
[20]: import statsmodels.api as sm

[~/statsmodels/statsmodels-skipper/statsmodels/sandbox/examples/]
[21]: aml = sm.datasets.get_rdataset("leukemia", "survival")                    

[~/statsmodels/statsmodels-skipper/statsmodels/sandbox/examples/]
\[22\]: aml.data.head(2)                                                          
[22]: 
   time  status           x
0     9       1  Maintained
1    13       1  Maintained

[~/statsmodels/statsmodels-skipper/statsmodels/sandbox/examples/]
\[23\]: aml.data.sum()
[23]: 
time                                                    678
status                                                   18
x         MaintainedMaintainedMaintainedMaintainedMainta...
```
