DataFrame should be more aggressive about trying to aggregate numeric data stored in dtype object
```
data = dict()
data['a'] =     [-0.00049987540199591344, -0.0016467257772919831, 0.00067695870775883013]
data['b'] =     [-0, -0, 0.0]
data['c'] =     [0.00031111847529610595, 0.0014902627951905339, -0.00094099200035979691]
AA = DataFrame( data , index = ['20110801', '20110802', '20110803'] , dtype = ‘object’ )
```
