add support for configurable canary api and checks hosts

There may be more elegant ways to do this and I'm open to suggestions. 

This assumes that you can simply drop a file called `canary-hosts.json` in the app root and have it served up by Apache or Nginx. I've only tried in the `public` folder on my dev box, but I think it is a safe assumption.

`canary-hosts.json` is assumed to look like this:
```
{
  "api": "http://mycanaryapi/",
  "checks": "http://mycanarychecks/" 
}
```

If `canary-hosts.json` is not found, then the hard-coded defaults are used.

If `canary-hosts.json` is present, then:
- `ENV.APP.CANARY_API_HOST ` is assigned to value of "api" property
- `ENV.APP.CANARY_CHECKS_HOST ` is assigned to value of "checks" property