The 'connection' accessor in collections and models actually refered to
a subclassed instance of Fog::Service which creates confusion in the
code.
References to 'connection' could have meant the service or the
Fog::Connection held within that service.
This deprecates the connection accessor and replaces it with `#service`
as a read only value. When a collection or model is initalised then
service should be passed.
This commit also updates all the changes to @connection made by
providers in model initialisers since these depending on the presence of
the 'connection' key. The key is still accepted by outputs a warning.
In many places we were checking for identity which was the shorthand for
checking if the resource had been saved by the service.
The #persisted? method was added to show a clearer intent and also offer
minimal ActiveModel interface
