Accept auth params via HTTP headers in more spec compliant way

We were previously allowing authentication params to be sent via their
own HTTP header fields, but the method was not particularly HTTP spec
compliant. This patch changes this to use the HTTP Authorization header,
which should be the correct fit for this purpose. When reading out
auth params, the library can now receive HTTP requests like this:

```
POST /feeds
Accept: */*
Connection: close
Content-Type: application/json
Authorization: ApiAuth api_ts="12345", api_hash="abcd12345"
Content-Length: 76
Host: leatherman.dev
```

And parse it out to a `HeaderAuthParams` object, which has corresponding
`api_ts` and `api_hash` methods. There's also a `to_h` method on the object,
which generates a simple hash that looks like this:

```
{
  api_ts: "12345",
  api_hash: "abcd12345",
}
```