Add Go To features for python

This PR adds a Go To Definition and Go To Declaration for python. Jedi has a 2 goto functions, they are [well documented](https://jedi.readthedocs.org/en/latest/docs/plugin-api.html#api.Script.get_definition). Basically, ``get_definition`` acts more like go to declaration, while ``goto`` acts like go to definition.

Since both methods can return multiple definitions we need to pass all of them to the user, so he can make a decision. This is done by opening a quickfix window with a ``youcompleteme#OpenGoToList`` method.