Add Gallery Styles

Patch for #425
Cleaned up gallery styles, per PR review
Prefix transient.
Remove html from string
cool-cats: only fetch the ids to limit footprint

We don't really care much about the returned; all we want is the count
so there's no point in fetching all the category data.
cool-cats: Limit number of categories fetched

We only need to know if there is more than one category so only fetch a
maximum of two categories. The rest of the categories are all gravy.
Merge pull request #445 from Automattic/speedy-cool-cats

_s: Optimize `_s_categorized_blog()`.
_s: Formatting adjustments.

Improves readability of argument array and brings new comment in line with all other comments in this function.
_s: Update pot file.

Last update was a good three months ago and we had some string changes
since then.

Fixes #434.
_s: Resolve merge conflicts from master branch.

This also retains `$all_the_cool_cats` for some flavor. It has been
around since the initial fork of Toolbox and makes people smile:
https://twitter.com/TheFrosty/status/449581069231071232
Merge branch 'pr/437'

Conflicts:
	inc/template-tags.php
_s: Update function return values.

Adds missing documentation for `_s_categorized_blog()` and removes void
statements, in accordance with WordPress documentation guidelines.

See
http://make.wordpress.org/core/handbook/inline-documentation-standards/p
hp-documentation-standards/#phpdoc-tags
_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The callback’s outdated documentation describes the function in an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.