show a warning when github is down

![screen shot 2014-10-08 at 5 31 13 pm](https://cloud.githubusercontent.com/assets/13248/4569529/bb5ec112-4f4b-11e4-9c25-5024ea4f8c4e.png)

Not really sure if this belongs in a helper or it's own class at this point, but whatever.

#29 

/cc @zendesk/samson @grosser 
Shouldn't this come from `Rails.application.config.samson.github.status_url` as well?