Go, Puppet, mode-line, git-gutter, fixes and tweaks

• New contrib layer go (thanks to rphillips)
• New contrib layer puppet added by a Puppet Labs engineer, classy! (thanks to hunner)
• New variable for the git layer: git-gutter-use-fringe which is t by default. See documentation (thanks to CestDiego)
• Fix lazy activation of package dependencies preventing successful byte-compilation of some packages.
• The left and right parts of the mode-line are now computed in two separate functions: spacemacs/mode-line-prepare-left and spacemacs/mode-line-prepare-right. It is now possible to add your own elements by advising these functions (thanks to roman)
• Ido key bindings fixes: C-x is now on C-o to visit a file in other window. Add M-RET to be able to open in dired in a terminal (thanks to danielwuz)
• Disable persp-cycle-(next/prev) functions for perspective layer. These functions are not working with the last version of perspective (thanks to CestDiego)