Allow use of Octopus with models with custom connection

So you can say:

```
class CustomConnectedModel
   octopus_establish_connection(...)
end
```

And then:

```
CustomConnectedModel.first # uses custom connection
CustomConnectedModel.using(:read_standby).first # uses read standby
```

For backwards compatibility & configurability, this only takes effect once a class attribute 'allow_octopus' is explicitly set on the model which has called octopus_establish_connection.
    
The use case is a model I have that lives in a separate database, which I do not normally want to have to think about when using it, but I would like to be able to call "using(:separate_db_standby)" with an Octopus-managed shard to access a read slave for the model.

Please ignore the branch name, which references an abandoned earlier approach.