Renamed iOS/Android project namespaces.

The Joey and Ross projects both had the XamarinStudio default
namespaces, which didn't include the "Toggl." prefix.
Refactor common IModelsView implementation into base class.

Having the observable properties in a base class enables us to have the
static fields with property names, which we can use in UI layers to
determine what changed on the OnPropertyChanged event, even if the
interface implementer isn't derived from ModelsView.