Add a primary mixin and remove IE8 support.

[notlee]

• Removes IE8 support (sorry: merged with this PR for convenience).
• Adds a primary mixin oGrid.
• Removes mixins and variables replaced by oGrid (see MIGRATION.md).
• Removes $o-grid-debug-mode in favour of calling oGridDebugInfo.

[notlee]

A future PR will revisit the README, including improving the oGrid entry.

[JakeChampion]

Bye Bye IE 8