API: relax categorical equality when comparing against object
from [SO](http://stackoverflow.com/questions/27164380/merging-pandas-dataframes-on-categorical-series)

```
In [1]: a = pd.Series(['a','b','c'],dtype="category")
In [2]: b = pd.Series(['a','b','c'],dtype="object")
In [3]: c = pd.Series(['a','b','cc'],dtype="object")
In [5]: a==b
TypeError: Cannot compare a Categorical for op <built-in function eq> with type <type 'numpy.ndarray'>. If you want to 
compare values, use 'series <op> np.asarray(cat)'.
```

```
In [6]: A = pd.DataFrame({'A':a,'B':[1,2,3]})
In [7]: B = pd.DataFrame({'A':b,'C':[4,5,6]})

In [9]: A.merge(B,on='A') 
Out[9]: 
   A  B  C
0  a  1  4
1  b  2  5
2  c  3  6

In [10]: A.merge(B,on='A').dtypes
Out[10]: 
A    object
B     int64
C     int64
dtype: object

In [11]: A.dtypes
Out[11]: 
A    category
B       int64
dtype: object

In [12]: B.dtypes
Out[12]: 
A    object
C     int64
dtype: object
```
