Add service alerts

[meringu]

Adds new service alert attributes to the sensor. For each trip:

• alert_count
• For each alert:
  • description
  • effect
  • header
  • severity_level
  • url

Alerts details are zero indexed, as there isn't a concept of a default alert like how the next service is the default trip.

I've only run this on my particular train route for a couple of alerts this morning. I don't have any alerts at the moment. Metlink should have some alerts affecting me tomorrow, so I can report back the results once that has happened.

[meringu]

This seemed to work pretty well for me this morning. I forgot to wire in the severity_level, so that has been done now.