Multiple themes support, better way to setup layer variables, Go, Elixir, DocView mode, tweaks and fixes

Important: This release introduces a breaking change: the variable name dotspacemacs-default-theme has been replaced for dotspacemacs-themes and the value is now a list. You can now define several themes, the first one is loaded when Spacemacs starts. You can cycle through your list with SPC T n

• Replace variable dotspacemacs-default-theme by dotspacemacs-themes. See documentation for more info
• Extend dotspacemacs-configuration-variables with properties, see documentation for more info
• Add alchemist.el to erlang-elixir layer (thanks to Ambrosia)
• Add go-autocomplete and company-go packages to go layer (thanks to Funnnny)
• Move all rainbow-xxx packages to the colors layer
• Add variable colors-enable-rainbow-identifiers to the colors layer. Default value is nil.
• Add vi like key bindings for doc-view-mode, see documentation for more info
• Add key binding SPC t h to toggle the highlight of the current line
• Add cython-mode to python layer (thanks to CestDiego)
• Add evil-anzu package (replace old spacemacs hack to integrate stock anzu)
• Fix scala-mode2 - Indentation error #383 (thanks to chrisbarrett)
• Fix the behavior of <tab> in term-mode buffers due to yasnippet (thanks to PythonNut)
• Remove definition of variable multi-term-program which prevented users from defining their own shell (thanks to t3chnoboy)
• Remove setting TeX-master variable in auctex layer (thanks to jmhammond)
• Disable hack for powerline height, see code
• First unit and functional tests of core with Travis CI setup (thanks to lazywithclass)
• Typo fixes and documentation update (thanks to jezenielzapanta, agsdot, dthg, fkautz, lionelyoung)