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.