Fix code completion not working.
Fix code completion not working.

A null reference exception was being thrown when the TypeSystemService
was trying to look at the project's OutputFileName. Now the DnxProject
overrides the DotNetProject's GetOutputFileName method and returns
null preventing the exception.

System.ArgumentNullException: Value cannot be null.
Parameter name: paths
   at System.IO.Path.Combine(String[] paths)
   at MonoDevelop.Core.FilePath.Combine(String[] paths)
   at MonoDevelop.Projects.DotNetProjectConfiguration.get_CompiledOutputName()
   at MonoDevelop.Projects.DotNetProject.GetOutputFileName(ConfigurationSelector configuration)
   at MonoDevelop.Ide.TypeSystem.TypeSystemService.ProjectContentWrapper.LazyProjectLoader.<.ctor>b__7e()

mrward/monodevelop-dnx-addin