Worked on trail pics a bit.

Wrote up a document explaining how we determine trail pics.
Merge branch 'master' of github.com:guardian/frontend into trail-pics
ignore breking codes
Merge branch 'master' of github.com:guardian/frontend
Merge branch 'master' of github.com:guardian/frontend
Fix test group
not using trailblock-show-more module anymore
Articles can load commercial components

Included is a simple commercial component loader, packaged and managed as part of our applcation but designed provide an sensible interface for
OAS/DFP to load our own adverts (based on AMD).

```
// load the 'travel' component
require('modules/commercial/loader', function (CommercialComponent) {
    new CommercialComponent(document, { config: guardian.config }).load('travel', document.getElementById('ad-slot'));
})
```
fix typos in ab-testing.md
fix typo in access-logs-for-your-localhost.md
fix typos in browser-support-principles.md
fix typos in commercial-components.md
XHR host name is now environmentally friendly
fix typos in interactives.md
fix typos in logging-client-errors.md
fix typos in sprite-production.md
fix typos in testing-externally-on-localhost.md
fix typo in commercial-components.md
Make generic fallback available for commercial components
Give default jobs if there are no targeted jobs to show