External traitlets

Work in progress. This does not commit (yet) the content of [this folder](https://github.com/ipython/traitlets/tree/master/traitlets) into `IPython/external/traitlets/_traitlets` still if you clone the traitlets module and symlink it or install traitlets IPython will work (shell start and test pass)


 - [ ] restart from scratch as some modification have been done into IPython itself.