Add `utils-offset` to development dependencies

Used in the grid tests.
Add new tests and spec new API

Include tests on grids with gutters applied and using `utils-dimension`
and `utils-offset` classes.

Change the modifier class names, include a test for right aligned cells.
Fix whitespace collapse; add modifiers

* Change `Grid--center` to `Grid--alignCenter`.
* Create `Grid--alignRight`.
* Use `font-size:0` to collapse inter-cell whitespace (no IE 8 support).

The new method for collapsing whitespace relies on `rem` support to
reset the font size in cells. IE 8 does not support `rem` units. If IE 8
support is needed, a preprocessor can be used to generate `px` value
fallbacks.
Remove test pollution

The `dev-Box` used to highlight cells was modifying the text alignment,
which was obscuring the `Grid` text alignment in the test.
Add `Grid--alignRight` test

Includes changes to the test headings to display the selector being
tested.
Update README documentation
Update CHANGELOGwq