API: Add str/dt accessors to categorical
Accessors should be enabled depending on `categories`. Should care `CategoricalIndex` also.

```
import pandas as pd
s = pd.Series(['A', 'B'], dtype='category')
s.str
# AttributeError: Can only use .str accessor with string values, which use np.object_ dtype in pandas

s = pd.Series([pd.Timestamp('2011-01-01'), pd.Timestamp('2011-01-01')], dtype='category')
s.dt
# AttributeError: Can only use .dt accessor with datetimelike values
```
