Add $mq-static-breakpoint to control which queries go in static stylesheets

Instead of simply outputting all min-width queries, we can now select a
target breakpoint, and only output queries that span (or start at) that
width.

This allows us to have breakpoints that are higher than our target
breakpoint for IE8 and friends (such as a 'widescreen' breakpoint)
without serving those queries to older browsers.
Update test suite to test $mq-static-breakpoint
Merge pull request #2 from bewildergeist/feature/static-breakpoint

Add $mq-static-breakpoint to control queries for static stylesheets