Refactoring of helper classes and more
- Rewrote layout helpers to use breakpoint loops and placeholders like the grid.
- Removed obsoleted helper mixing from mixins/_helpers.scss

Minor tweaks
- Changed “large-tablet” to “ipad”, conforming to the original code
- Changed “$default-breakpoint” to “$default-base”, and had it automatically default to the last item.
- Added “no-border” to definable list of $border-helpers
- Made grid/multi-column mixins more robust, able to accept empty strings and bools in a standard fashion.