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 #13; move default threshold into Stoplight module
Create pending shared example group for data stores
Fix #8; implement shared data store examples
Group examples into contexts
Remove redundant examples
Add mixin example
Update changelog
Merge branch 'issue-4'
Update changelog
Merge branch 'issue-5'

Conflicts:
	CHANGELOG.md
Use forwarded data store methods
Re #7; create a basic notifier class
Create .notifiers method for configuring notifiers
Notify when moving from green to red
Create HipChat notifier
Update changelog
Update changelog
Document notifiers