Customizable command key, better ESC and Bug fixes

• New variable dotspacemacs-command-key to customize the ex-command and M-x command key bindings. Default is :.
• Improved ESC in minibuffer, it is now easier to close the minibuffer with the escape key (thanks to avendael)
• Fix initialization: Packages and extensions are now correctly loaded in alphabetical order.
• Fix #223: auto-centering of buffer when buffer-menu is opened (with :buffer or C-x C-b)
• Fix #237: Bug on OS X which prevents Emacs from really deleting files
• Fix #239: goto-next-section and goto-previous-section now works in Magit buffers, respectively on C-n / C-j and C-p / C-k.
• Fix #220: Missing description in guide-key buffer
• Fix typo in python layer: pyenv instead of pyvenv (note the additional v!) (thanks to danielwuz)