Allow headings to be overriden per story

Eg. Background -> "How did this happen?"
In article video poster image

Poster image is now woven into the video. This is at the mercy of whatever picture comes out of the API as the 'still image' field. We might want to have a chat with the people who upload them.

If the video is the first thing inside the body (i.e. the very first HTML element inside the body) then the main picture will be hidden.
Merge branch 'master' of github.com:guardian/frontend into in-article-video
Merge branch 'master' of github.com:guardian/frontend into in-article-video

Conflicts:
	data/database/0ca267a5a9906e7988de0c7a413ad4bd3c48a4ed
Avoid forcing a new composited layer for no reason.
Merge branch 'master' of github.com:guardian/frontend into in-article-video

Conflicts:
	data/database/b3aa5b3f03ee1a6abbb0e49a22ffeb68ae6ce7b9
Merge branch 'master' of github.com:guardian/frontend into in-article-video

Conflicts:
	data/database/b3aa5b3f03ee1a6abbb0e49a22ffeb68ae6ce7b9
Merge branch 'master' of github.com:guardian/frontend into in-article-video
Switch back to previous version of grunt-contrib-sass
Moved blocking code out of plugin onStart for football tests.

Play does not like blocking here
Merge pull request #609 from guardian/football-tests

Moved blocking code out of plugin onStart for football tests.
Ensure test data is loaded
Give football data a bit of time to load
Merge pull request #603 from guardian/in-article-video

In article video
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.