Work on making the REPL completion show names instead of dots

This is NOT ready to be merged. I am just opening a pull request so it is easier to see the code.  See https://github.com/davidhalter/jedi/issues/295.

So far:

- [ ] This will need to use the `rl` library. I believe that provides sufficient functions to make this really easy.
- [ ] I am using a regular expression to get the "completed" part out of the input, but surely Jedi provides an API for this. 

I think I know how to do the first point (if I'm not mistaken, http://pythonhosted.org/rl/readline.html#rl.readline.get_completion_display_matches_hook should help), but I'm not sure about the second. Any suggestions there would help.