ENH: Allow read_csv dtype to accept category
Like R's `stringsAsFactors`, it is nice to create `category` when read data. Currently, it raises `TypeError`.

```
import pandas as pd
import StringIO
pd.read_csv(StringIO.StringIO("""A, B
a, c
b, d"""), dtype={'A': 'category'})
# TypeError: data type "category" not understood
```
