Clean up language in DSL and log output

There are a few highlights here:

- `childs` has been changed to `children`, across the board. This changes two elements in the DSL (`children_update_period` and `stop_children`, now), but I think breaking the API slightly is worth the added internal consistency (we were already using `monitor_children`, for example).
- I've attempted to clean up the grammar and format of the logger output throughout eye. See https://github.com/ahorner/eye/commit/e9dd28775b053614fea799779d6979f85c5181e3 for more details.
- I replaced a few "realize me" raises with proper `NotImplementedError`s.

My team is using eye successfully in production environments for a growing number of projects now; thanks for building such a useful gem.