Add `main` element to HTML5 display definitions

The HTML Working Draft has added `main` as a new element.
Update CHANGELOG
Remove unnecessary `pre` styles

`word-wrap: break-word` was only needed for IE 6/7, and `white-space:
pre` is the default for all supported browsers.

Fix gh-159
Add `hr` test
Normalize `hr`

Firefox uses different `box-sizing` and `height` values to all other
browsers. Firefox doesn't currently support `box-sizing` without the
`-moz-` prefix, so we use both the vendor-prefixed and unprefixed
properties to ensure that it matches the `content-box` value of other
browsers. It also requires the `height` to be set to `0`.

Fix gh-133
Update CHANGELOG
Normalize `h1` margin

Chrome, Safari, and Firefox all adjust the margin of `h1` at several
levels of nesting within HTML5 sectioning elements. This change ensures
that the margin, like the font-size, does not vary in these contexts.

Fix gh-160
Normalize form element `text-transform`

By default, browsers set `text-transform:none` on most form controls in
order to prevent `text-transform` being inherited from ancestor nodes.

However, the `button` and `select` elements are exceptions.

* Firefox and Opera do not apply `text-transform:none` to `select`.

* Chrome, Safari, and IE 8+ do not apply `text-transform:none` to
  `button.

It's not suitable to set `text-transform:inherit` because all other form
elements intentionally avoid it. Safari will not honour that style for
`select`, and Chrome will only do so when the `select` element is
clicked.

Further details:
http://tjvantoll.com/2012/07/10/default-browser-handling-of-the-css-text-transform-property/
Update CHANGELOG
Update README acknowledgements
v2.1.0
Add Bower install and more to README

* Mention v1's support for legacy browsers.
* Provide link to project page for easy download.
* Include Bower install command.
Clean up component.json

Bower doesn't currently use several properties that were included.
Keep `styles` for now, for Component(1).
Protect root against system color schemes

System color schemes (in particular, custom themes in Windows and Linux
distros) affect Firefox, IE, and Opera. Normalizing the web site/app's
root background and text color prevents these unwanted settings from
being used. Chrome doesn't apply system-level schemes to websites.

Fix gh-170
Fix typo
Change gh-page URLs to use `.io`

GitHub have redirected all `username.github.com` pages to
`username.github.io`.
Add contribution guidelines

Includes process for maintainers while there is no automation in place.

Close gh-175
Fix example `git tag` command in docs
v2.1.1
Fix typo in CONTRIBUTING.md