Added the CSS output for the first example.

Inspired by this SO questtion http://stackoverflow.com/questions/21853647/ (see comments there). I.e. it's not that evidend what result it will be when e actually mix the mixin in.
mixins.md: fixed typo "hen" -> "when"
online-less-compilers.md: added "JS Bin".
Merge pull request #114 from seven-phases-max/master

online-less-compilers.md: added "JS Bin".
extend.md: minor Duplication Detection example fix.

#113
added Vietnamese translation link.
Documented `replace` function (initially).
Merge pull request #115 from seven-phases-max/master

Documented `replace` function.