Initial providers abstraction.
Adding provider interfaces.
Making servers_test pass with mock object.
Adding in some struct for Image, Location, Size.
Updating vbox provider.
Fixing up servers_test.
Adding some tests and implementations for `vbox/servers`.
Adding Servers.Show to vbox provider.
Updating provider interfaces.
Passing all the vbox server tests.
Merge pull request #2 from gcloud/providers

Adding Providers. Virtual Box is the first.