update_rate parameter uses TimeValue instead ms

Until version 1.2.0, `update_rate` parameter was using only ms as a value.

We can now support ms but also `TimeValue` format such as `10s`, `1h` or so.

Closes #83.

Empty fs river definition throws an error

A recent changes ( #12 ) broke the default empty river:

```
{
  "type": "fs"
}
```

Closes #84.