* Remove IAssemblyProvider.
* Remove DefaultAssemblyProvider in favor of DefaultAssemblyPartDiscoveryProvider.
* Update AddMvcDnx to add the list of DNX discovered assemblies to the list of application parts.
* Added ControllerFeature and ControllerFeatureProvider to perform controller discovery.
* Changed controller discovery to use application parts.
* Changed ControllerActionDescriptorProvider to make use of Application parts.
* Simplified AddControllerAsServices to not accept any parameter and perform
controller discovery through the ApplicationPartManager in the IMvcBuilder
and IMvcCoreBuilder. Assemblies should be added to the ApplicationPartManager
in order to discover controllers in them.