#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 fix doesn't feel quite right. Ideally, we should ensure that things are called with the right `tb_offset`, not try to guess what it should be in here. Hopefully I'll understand how to do that better when I've read more of this.