RESTclient, default theme support, fixes and tweaks

• New contrib layer restclient (thanks to CestDiego)
• Add support for custom theme with a new variable dotspacemacs-default-theme for .spacemacs, see documentation.
• helm-projectile-ag support (thanks to rphillips)
• new helm bindings <SPC> h r to resume the last helm buffer.
• Fix <SPC> p D binding for projectile-dired
• New key bindings for hl-anything and move .hl-save file to .cache directory.
• Fix directory structure for org-repo-todo layer (thanks to CestDiego)
• Smarter smartparens (thanks to trishume)
• Fix projectile-persp-bridge init of perspectives layer (thanks to CestDiego)