Add $mq-show-breakpoints to show active breakpoint
Fix indentation + boolean + switch to body:before
Change $mq-show-breakpoints from boolean to list

This allows $mq-breakpoints to be sorted by something other than width.
For the breakpoint debugging to work, $mq-show-breakpoints must still be
sorted sequentially by breakpoint width.
Merge pull request #7 from bewildergeist/feature/show-active-media-query

Ability to show active breakpoint in corner of viewport