Simple health status middleware

This is a simple middleware that can be used to return app details. The payload looks something like:

```
{
    "name": "user_facts_service",
    "hostname": "Darbys-MacBook-Pro.local",
    "revision": "b85730f7e879f6f506183d1b1e803b8c6271f3d4",
    "pid": 27304,
    "parent_pid": 27302,
    "platform_revision": "0.0.14"
}
```

I can see this being expanded to include backend checks, etc. Something like:

```
{
    "name": "user_facts_service",
    "hostname": "Darbys-MacBook-Pro.local",
    "revision": "b85730f7e879f6f506183d1b1e803b8c6271f3d4",
    "pid": 27304,
    "parent_pid": 27302,
    "platform_revision": "0.0.14"
    "mongodb": true,
    "redis": false
}
```

Still need to work out the details on that, but should be doable.

@jdoconnor @kreedy @christianvozar