Added new Flexbox mixins from https://gist.github.com/cimmanon/4461470
Added import for new Flexbox mixins
Added tests for Flexbox
Converted prefix support variables to match the new style
Corrected behavior for  argument on  mixin
Reorganized comments & default variables
Fixed supported prefixes order and function call (was using incorrect name)
Adjusted function name and added missing commas
Added examples/documentation
Added missing elipsis
Merge pull request #1218 from cimmanon/master

New Flexbox Mixins