Merge tag '2.7.0' into develop

2.7.0
Add operamini and cutsthemustard utils. Add corresponding root classes. Closes #241
Merge remote-tracking branch 'origin/develop' into develop
Refactor respond to mixin and grid system.

Major refactor to simplify respond-to mixin. Now expects a breakpoint
variable rather than a string. Also now relies on variable to determine
at which breakpoint it should stop serving styles to IE. Avoids oldie
getting styles designed for widescreens.
Added prepartory placeholders for enqueuing conditional comment styles via WP.
Update version and changelog
Merge branch 'release/2.8.0'