Better font support, Magit SVN, New supported themes, tweak and fixes

• Better support for default font with a new variable dotspacemacs-default-font, see documentation for more info
• Easy tweak of powerline height via the property :powerline-offset of variable dotspacemacs-default-font, see documentation for more info
• Better fallback fonts
• Correctly set the desired font size of both default font and fallback fonts
• Add support for a bunch of non conventional theme packages (thanks to Ambrosia)
• Add magit-svn plugin to git layer. Enable it with the layer variable git-enable-magit-svn-plugin.
• Move rainbow-delimiters back into spacemacs layer since it is a useful package to have by default to edit elisp code (thanks to trishume)
• Fix a critical bug where cancelling Emacs exit had no effect