refactor(tests): own folder for each type of mixin / feature

-- own assets and html-file for each feature
feat(animations): add animations as mixin
feat(completions): included some various completions, which compass hadn't get

-- user-select
-- placeholder
feat(flexbox): added cross-browser flexbox-styles
feat(svg): mixin for svg-fallback
feat(fonts): include special fonts via mixin

-- use @mixin font-face($name, $url) to implement a font
chore(grunt): watch assets of each tested module
refactor(tests): removed global assets for tested files
chore(grunt): changed path to scss-files
fix(svg): changed the order of background-images
chore(tests): add headlines