Refactor CSS files for podlove-webplayer into SCSS with SASS and Compass

- add Compass config.rb to define paths and the output style for preprocessing css files
- create sass directory for scss files
- create partial scss files for modules, basic settings and components for modularization
- refactor basic styles from old css file into partial files
- import partial files in podlove-web-player.scss
- delete refactored styles from podlove-web-player.css
Move mediaelement styles to partial file and compress lines using scss
Move styles to partial files

- add _theme.scss for specific Podlove styles
- add _button.scss for buttons, _control.scss for controls
- add _utilities.scss for general helper classes like 'hide' 'hidden', 'clearfix',...
- modify other partial files while working in progress (move old styles to partial files)
Merge branch 'embed' of https://github.com/podlove/podlove-web-player into preprocess-css
Configure debugging statements in config.rb

- Import compass plugin 'sass-css-importer'
- Disable line-comments in css output

Import 'sass-css-importer'
Refactor css

- Import medialement.js into _mediaelement.scss
- Import podlove fonts to podlove-web-player.scss
- Delete _icons.scss and _fonts.scss, they weren't used because of the imports
- Move all theme styles to _theme.scss
Add production css output-file

- Compile to compressed (minified) output css
Update readme file

- Create chapters 'Build CSS', 'Setting up the ruby environment', 'Install SASS', 'Install Compass', 'Build the CSS file'
- Create the chapter 'Contributing' to encourage and help new contributors
Merge pull request #149 from plutonik-a/preprocess-css

Preprocess css