Option to require authentication to access meters

*Credit: This implementation copied directly from the [rails_admin engine](https://github.com/sferik/rails_admin)*

Add a configuration option to authenticate users who want to view meters

**TODO**

I was unable to run the tests because there is a problem setting up the test db:

```
> RAILS_ENV=test bundle exec rake db:migrate
rake aborted!
Multiple migrations have the name CreateMeterCatMeters
```