Fix println! spacing, clean up comments
The '+'/'-' usage was confusing so I split the lines. In addition,
I amended println! to conform to the 4 spaces for indentation as
stated in the style guide.
Add Playground button
Add Font-awesome output button to result div
Take Editor contents and run them in the Rust playground
Modify comments to add/subtract/multiply/divide, remove playground button
Remove the replacement of line breaks by putting output button first
Replace font-awesome css with fontello svg font
Modify HTML link from editor.js, add playground link on error
Merge pull request #35 from jasonkliu/14652_frontpage_style

14652 frontpage style