Improve error reporting from async callbacks

Implement helper Callback classes that report errors to AsyncTestCase.

Clean up tests to use these new helpers.

Bump up the strong-remoting-android dependency to 1.1.1-SNAPSHOT to
bring in the fix in RestAdapter adding a call to onError when onSuccess
throws.
Code cleanup: extract field REST_SERVER_URL

Extract the URL of the test REST server into AsyncTestCase superclass.
Implement Installation Model & Repository.

Add a helper method `InstallationRepository.createAndroidInstallation()`
that pre-fills properties like `appVersion` and `status`.
Merge pull request #18 from strongloop/feature/add-push-notification-models

Add push notification models