Make convert_objects less aggressive
I find the following highly counterintuitive:

``` python
df = DataFrame([["2003-1-1", "1"]], columns=["a", "b"])
conv = df.convert_objects(convert_numeric=True, convert_dates="coerce") 
conv.b.dtype == dtype('<M8[ns]')
```

I think date and numeric conversion should be independent between columns and dependent between rows, since DataFrames are column oriented. Maybe numeric conversion should be tried first and then if a `TypeError` or some other more appropriate exception is caught then try date conversion.
