Add Gallery Styles

Patch for #425
Cleaned up gallery styles, per PR review
Remove html from string
Add support for html5 caption
_s: Encourage the use of sidebar descriptions.

Sidebar descriptions provide an opportunity to provide context about
where the sidebar is located or when it is loaded. We should encourage
theme developers to use them, similar to how we hint default images for
custom headers and backgrounds.

The callback’s outdated documentation describes the function in an
early state of Toolbox (introduced in r3678-wpcom-themes), the earliest
public record would be
https://themes.trac.wordpress.org/browser/toolbox/0.3/functions.php#L99
This is as good an opportunity to update it as any.

Props @emiluzelac for initial patch.
Fixes #433.
_s: Simplify gallery styles.

It consolidates the duplicate `.gallery` selectors, and removes image
styles that needlessly override their earlier defined styles.

Also switches to use inline-block over floats, and adds documentation
for caption styles, to be coherent with documentation for gallery
styles.
a11y improvements on primary navigation menu toggle
Merge pull request #458 from Automattic/pr/429

Updated gallery styles to work for both HTML4 and HTML5 versions.

Fixes #425.
Closes #429.
Removed addEvent support

As noted in #456, IE8 is no longer supported.

Also, undefined is falsey in JavaScript, so the typeof check was unnecessary.
Added contributing guidelines

https://github.com/blog/1184-contributing-guidelines
_s: First version of contributing guidelines.

This might change in the future, based on our experiences, but should
be a good first draft of helpful and friendly guidelines for
contributing.

Props @bungeshea.
Fixes #381.
Merge pull request #461 from jefferyto/patch-2

Removed addEvent support.

See #456.
Remove style redundancy for menu toggle; cursor pointer is already defined for buttons.
Add tab to indent 404.php(L43 to L45).
_s: Add context for translations and styles.

The referenced items are decisions we made for `_s`, that a re likely to be brought up again in pull requests or issues. Since our stance on them is not likely to change any time soon, let's include them in the Contributing FAQs.

See #464.
Merge pull request #463 from gatespace/formating-source-code

Indent multiline PHP code.
Merge pull request #443 from joshmcrty/master

Add support for HTML5 captions.
_s: Simplify caption styles to work in HTML5 mode.

Since HTML5 captions removed the additional 10px that HTML4 versions
come with, the `max-width: 98%;` rule made images blurry. This slightly
simplifies caption styles while still providing support for both HTML4
and HTML5 versions.

See #443.
Merge pull request #459 from philiparthurmoore/menu-a11y

a11y improvements on primary navigation menu toggle
Update _s.pot : add "Primary Menu" in header.php(L31).