Add save/load functionality and fix issue with sorted

[jhmenke]

• Implemented simple save/load of a fitted encoder by using pickle
• sorted was removed in a commit, which let the tests fail due to random orders of the column_mapper variable. a custom sort function is implemented to deal with mixed sets (e.g., {nan, "a", "b", "c"}).

[jhmenke]

also fixes #10 now

[yashu-seth]

@jhmenke Thanks a lot for this. I request you to please make a single change in one commit. I see that the "fixed tests" commits are also making changes in the code and not only in the test. Also, sort_maybe was first added and then replaced by sort_mixed. Can you please add the final changes represented in separate commits? This would help us in keeping the commit history clean. You can force push/ rebase these commits to make the commits history more organized.
Let me know if you need more clarification.

[jhmenke]

okay, no problem. might take a few days though to find the time

[jhmenke]

i finally got around to it and repackaged the changes as a single commit with a descriptive message

[yashu-seth]

@jhmenke I have a few suggestions. Sorry, I am not getting time to post a detailed review. Will get to this ASAP.