#1673 Display of chained exception

I changed displaying exceptions to support chained exceptions in python3. It fixes [#1673](https://github.com/ipython/ipython/issues/1673).

Nothing changed for python2. However I did some refactoring in `ultratab.py`, such us PEP8 fixes and extracting code to smaller methods (major function for formatting traceback was ~300 lines long and rather ugly).

I created two tests for the new behaviour.

Here is how it looks like in notebook:
![Imgur](http://i.imgur.com/W69oHuU.png)
This logic seems to assume that an exception without `__context__` can never have `__cause__`. I admit I've only skimmed through [PEP 3134](http://legacy.python.org/dev/peps/pep-3134/), but I think it's possible, though unlikely, to have `__cause__` without `__context__`.