Button component – Normalize border-radius across element types in iOS

In iOS, by default `<input>` elements have a small border-radius while `<button>` and `<a>` do not. This makes the border-radius 0 for all types.

Status: **Once #32 is merged, ready**

Reviewers: @kpeatt @jeffkamo

## Changes
- Sets border-radius to zero for all button components. Value can be overridden on a project basis.

## How to test-drive this PR
- Run `grunt` and open `dist/components/button/index.html` in iOS