Equal-width modifier layout calculation doesn't work as expected/described

As described in the docs, `.Arrange--equal` should create "equal-width cells". Through some trial and error, I've found that cells are not equal width if the width of content within them isn't uniform. It looks like what's actually happening is that the width *between cell content* is equal, but the width of cells is not.

Applying `width: 100%` to `.Arrange--equal` makes cells actually equal width, although I don't think I understand the table layout algorithms well enough (or at all) to know why that is.

I made a couple of gifs toggling that rule for comparison.

In the test suite:

![arrange](https://cloud.githubusercontent.com/assets/808159/5079904/2a4a16a8-6e6e-11e4-90f0-1a26ea7a3a26.gif)

On Twitter:

![twitter](https://cloud.githubusercontent.com/assets/808159/5079906/2b68746c-6e6e-11e4-8afd-1f19f4e89ca7.gif)