Add Gallery Styles

Patch for #425
Cleaned up gallery styles, per PR review
_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The callback’s outdated documentation describes the function in an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.
Merge branch 'master' into ja

* master:
  _s: Encourage the use of sidebar descriptions.
update POT files.
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.
Merge remote-tracking branch 'upstream/master' into ja

* upstream/master:
  _s: First version of contributing guidelines.
  Added contributing guidelines
  Removed addEvent support
  _s: Simplify gallery styles.
  Cleaned up gallery styles, per PR review
  Add Gallery Styles
add japanese CONTRIBUTING-ja.md
Merge remote-tracking branch 'upstream/ja'

* upstream/ja:
  add japanese CONTRIBUTING-ja.md
  _s: First version of contributing guidelines.
  Added contributing guidelines
  Removed addEvent support
  update POT files.
  _s: Simplify gallery styles.
  _s: Encourage the use of sidebar descriptions.
  Cleaned up gallery styles, per PR review
  Add Gallery Styles