Add unit tests for zen-grid-background().
Zen Grids 2.0.0-alpha.1 is a major refactoring of Zen Grids.

Changes since 1.3:
- Add bower component.json.
- Add zen-grid-background() mixin.
- Move zen-grid-flow-item() and zen-float() to a new zen-grids/flow module.
- Use simplified -100% margins.
- Change @import "zen" to @import "zen-grids" to make installation more natural.