Add Gallery Styles

Patch for #425
Cleaned up gallery styles, per PR review
_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 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.