Read_csv should raise error on bad dtypes
OPs code from http://stackoverflow.com/questions/16988526/pandas-reading-csv-as-string-type

Add: auto conversion from a passed `str` or `np.string_` to `np.object` might be ok

```
df
           A         B
1A  0.209059  0.275554
1B  0.742666  0.721165

df.to_csv(savefile)

df_read = pd.read_csv(savefile, dtype=str, index_col=0)
   A  B
B  (  <
```

Actually I get something different on dev:

```
In [101]: pd.read_csv('a', index_col=0, dtype=str)
Out[101]:
 A B


```
