Return bonafide model objects where possible for nested attributes

[djberg96]

For example, if I want to get a list of network interfaces for a virtual machine I could do this:

vm.properties.network_profile.network_interfaces.each{ |n| ... }

The problem is that the objects returned by that only have the :id attribute because that's what the JSON has. However, we have NetworkInterface model objects as well.

So, instead of just returning a simple object with an :id field, provide a generic method to call that returns a real model object if we have one that wraps it. The :id field should still exist, too, in case users want to use the original value. So, something like:

vm.properties.network_profile.network_interfaces.each{ |n| n.model ... }

[djberg96]

Maybe an Enumerable#each_model method?

[djberg96]

Another option would be to either modify the ResourceGroupBasedService#get method to accept an id string, or to provide a find_by_id method that smartly determined which Service#get method to call.

[djberg96]

To that end I've submitted this PR: #225