Allow dependencies to specify the client running the check

A simple change to the format of dependencies entries, allowing checks to be prefixed by 'client'/ so that checks on one client can be depdendent on the results of checks on another e.g:

```
{
  "checks": {
    "banner": {
      "command": "/etc/sensu/plugins/network/check-banner.rb",
      "dependencies": ["foo.example.com/keepalive"]
    }
  }
}
```

I'm not sure if '/' is the best separator to use, or even if there isn't a better way to split the check field to extract the client, but this can be considered a 'proof of concept' if nothing else.