Python contrib

Not fully complete yet, but just wanna get early feedback from you and whoever interested in making writing python code more pleasure with `spacemacs`.

I've done below items so far(at this point of this PR):
- Moved python related setup in `spacemacs/packages.el` to dedicated contrib
- Use anaconda instead of jedi.
- Added `eldoc` to see realtime documentation whilte coding
- Added `pyvenv` for virtual environment

I just have few questions for you here,
- how should I name this contrib? I initially named it `contrib/lang/python/`, but then I realize with that naming convention, I need to call the initialization function `python/init-python`. Concerned about name conflict, I'm currently using name `spacemacs-python`
- what makes the call of adding a package as dependency or extension? I've noticed that currently [`nose`](https://github.com/syl20bnr/spacemacs/tree/master/spacemacs/extensions) package is been added as an extension, but it's also available on melpa. Just wondering in which case I should make a module an extension but not dependancy(from elpa/melpa)

Couple TODO items:
- move `pylookup` over
- move `nose` over
- separate key bindings to keybindings.el
- setting python shell interpreter and break point based on the availability of `ipython` and `ipdb`

Let me know your thoughts on the changes I've made so far.