Fixes #3156:Categorical.sort_values bug

[stress-tess]

This PR fixes #3156, Categorical.sort bug, changes the name to sort_values, and adds testing

[ajpotts]

Looks great!

Would you be willing to change the name of this function to sort_values to match pandas?

In [32]: c = pd.Categorical(["a","b","a","c"])

In [33]: c.categories
Out[33]: Index(['a', 'b', 'c'], dtype='object')

In [34]: c.sort_values()
Out[34]: 
['a', 'a', 'b', 'c']
Categories (3, object): ['a', 'b', 'c']

[stress-tess]

Looks great!

Would you be willing to change the name of this function to sort_values to match pandas?

In [32]: c = pd.Categorical(["a","b","a","c"])

In [33]: c.categories
Out[33]: Index(['a', 'b', 'c'], dtype='object')

In [34]: c.sort_values()
Out[34]: 
['a', 'a', 'b', 'c']
Categories (3, object): ['a', 'b', 'c']

sure! that sounds like a good idea