Csharp, Django, ESS, Toggle Framework. Yasnippet, tweaks and fixes

• Add new contrib layer csharp (thanks to bjarkevad)
• Add Django support via pony-mode for python layer, see documentation for more info (thanks to BrianHicks)
• Add ESS noweb (knitr/sweave) keybindings to ess layer (thanks to jcpetkovich)
• Update ess layer key bindings according to the conventions, see documentation for more info
• Introduce a framework for toggles, no documentation for now but it can be tested by invoking helm-spacemacs with SPC f e h then going to the next tab with C-l (reminder: SPC h l resumes the last helm buffer) (thanks to trishume for the idea)
• Add a new default directory for yasnippet snippets in private directory. This directory is automatically loaded by yasnippet, this is the perfect place to put user specific snippets (thanks to ralesi)
• New variable for clojure layer clojure-enable-fancify-symbols. Default value is nil, see documentation for more info (thanks to oppenlander)
• Better ex-command :nohlsearch which now works as expected (i.e. like in Vim) (thanks to kini for the report)
• Fix regression with fonts in Cygwin Emacs version (thanks to mijoharas for the report)
• git-gutter-fr:side variable can now be correctly overridden in .spacemacs (thanks to jstokes for the report)