Remove zen-nested-container() by merging it into zen-grid-container().

Add $context parameter to zen-grid-container() so it can be used: without
context, on grid items, or on elements in the normal flow inside a grid item.
Update README.
Zen Grids 2.0.0-alpha.2 is the next release of a major refactoring of Zen Grids.

Changes since 2.0.0-alpha.1:
- Remove zen-nested-container().
- Add $context parameter to zen-grid-container() so it can be used: without
  context, on grid items, or on elements in the normal flow inside a grid item.
- Add proper Sass 3.2 dependency.

Changes since 1.3:
- Zen Grids is now also a Bower component.
- Add mixin for grid background images for use durin development.
- 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.