Added images for patterns from #311.
Moved globals directory into helpers/main.

Helpers should not generate any CSS (used by helpers.scss). Font
declarations therefore belong in main. This brings structure in line
with main DS app.
Group polaroid pattern with other image styles.
Remove extraneous cursive font fallback.
Added KSS documatation to message callout
Fixed image URLs in message callout.
New helper asset URLs and new patterns.
Added @weerd's container pattern.
Updated style guide with new patterns.
Updated heading styles from container.
Use box-sizing mixin rather than generic prefixer.
Merge pull request #312 from DoSomething/pattern-docs

Pattern docs