TODOS is completer.py and class enhancements

There are two "TODOS" in [IPython/core/completer.py](https://github.com/ipython/ipython/blame/master/IPython/core/completer.py):

- Line 1902: supress an if clause
- Line 1956: FOLD the 3rd throw away argument of _complete into the first 2 ones

I'd like to work on this if possible, but I have a few doubts:

1) I don't understand what the if clause on the first line is testing and what the behavior expected here;
2) The second line seems simpler, I just wanted to check whether we're supposed to change the "_complete(..)" function definition or the returned constructor on the function where the "complete(...)" function.

I also think that this file is a little bit too long and because of that I think it's a little bit hard to understand all the classes and function purposes. I wanted to volunteer to rewrite it if possible.

Thanks!