Add Grid component with tests.

Add component for building simple n-up grid layouts, optionally with gutters. Allows single or multiple grid rows. Asymmetrical and complex grids can be created with component-level and/or item-level modifiers. 

Status: **Opened for visibility**
Reviewers: @jeffkamo @kpeatt @scalvert 

## Changes
- Add Grid component
- Add tests

## Notes
- Since this uses floats, rows of unequal items must be cleared with `nth-child` for each column configuration. If we could switch to `inline-block`, this inflexibility would go away (poor `flex-wrap` support means Flexbox is not an option). However, Android pre-4.1 has issues with removing whitespace between inline-block elements. If a solution can be found, this should be re-implemented using `inline-block`.