Allow headings to be overriden per story

Eg. Background -> "How did this happen?"
Avoid forcing a new composited layer for no reason.
Merge pull request #608 from guardian/rendering-perf-fix

Avoid forcing a new composited layer for no reason.
Update README.md

adding link for external people to get API keys
Do speed detection with responseEnd, so it's measuring more than just latency.
Merge pull request #610 from guardian/tweak-speed-detect

Do speed detection with responseEnd, so it's measuring more than just latency
Fixing Jasmine tests.
Merge pull request #611 from guardian/tweak-speed-detect

Fixing Jasmine tests.
Report unknown connection speeds rather than the assumed high value.
Added stale-while-revalidate and stale-if-error headers
Set stale-if-error to 45 days
Merge pull request #601 from guardian/heading-as-labels

Allow headings to be overriden per story
Merge pull request #615 from guardian/tweak-speed-detect

Report unknown connection speeds rather than the assumed high value.
Merge pull request #614 from guardian/stale-headers

Added stale-while-revalidate and stale-if-error headers
Added witness Call To Action