Fixed Deprecation in PHP6

Accessing Strings via Curly brackets will be deprecated in PHP6
http://php.net/manual/de/language.types.string.php#example-58 (above this example)

also i don't think it is as readable as brackets
Merge pull request #400 from Handfeger/master

Using square brackets instead of curly brackets to access string content
Fix typo in docs/docs.md

constrast -> contrast
Merge pull request #406 from TimWolla/patch-1

Fix typo in docs/docs.md
Adding viewport directives
Adding to directive tests
Merge pull request #407 from AnhNhan/master

Adding `@viewport` directive and relatives
don't mixin self when calling without args, fixes #412 #405
Made the third parameter of mix() optional to match less.js

Closes #425
Merge pull request #427 from Incenteev/lessjs_mix

Made the third parameter of mix() optional to match less.js
Merge remote-tracking branch 'upstream/master'