Guide-key, evil-nerd-commenter and fixes

• evil-nerd-commenter is now working for 24.4 (https://github.com/syl20bnr/spacemacs#commenting)
• guide-key (thanks to ralesi) and guide-key-tip support (enabled by default, can be toggled with <SPC> t G) with golden-ratio compatibility (in a graphical env. guide-key is displayed in a tool tip, in a terminal it is displayed in a buffer)
• key binding for helm-projectile is now <SPC> p h