Added a simplified emacs configuration

This is a distilled version of my old config, after 6 years adding different
snippets found all over the Internet became harder and harder to maintain,
so I started with a clean config and I'm adding only the bits I really need
also porting old code to new emacs ways of doing the same, specially the way
the pacakges are installed.
Added virtualenv and nose emacs packages
Added yasnippet and gruber-darker-theme

Also marked gruber-darker-theme as safe to load
Deleted trailing whitespaces
Added several changes related to python
Added 'git tip' alias to mimic 'hg tip' command
Merge branch 'new-emacs'