Fix #4; create with_light mixin

Instead of this:

    Stoplight::Light.new('example') { something }.run

You can do this:

    include Stoplight::Mixin
    stoplight('example') { something }
Fix #5; rename NoFallback to RedLight
Fix long line
Add link to stoplight-admin
Add mixin example
Update changelog
Merge branch 'issue-4'
Update changelog
Merge branch 'issue-5'

Conflicts:
	CHANGELOG.md
Use forwarded data store methods
Merge branch 'master' into issue-13

Conflicts:
	CHANGELOG.md