Remove flexbox to correct misaligned text. Fixes #26.

(brief description / PR title)

Status: **Ready to merge**

Reviewers: **(GitHub @<name>s of all reviewers)**
Ticket: Fixes #26

## Changes
- Removes flexbox properties from button component. Note that this causes vertical alignment to differ between input, button and anchor elements only when the text within the button wraps. In this case, the solution is to apply `vertical-align: middle`. However, in the common case of non-wrapping button text the preferred alignment is baseline, so this remains the default.
- Update comments.

## How to test-drive this PR
- Run `grunt` and open tests/components/button/index.html