dnu restore log should indicate when postrestore and prepare scripts start getting excecuted

[balachir]

1. New ASP.NET 5 project from StarterWeb template
2. View package restore log

EXPECTED
Package restore log should contain entries to indicate when postrestore and prepare scripts start getting executed. This will make it more intuitive to the user that these scripts in project.json are part of the package restore, and also give them a clear idea regarding which part of the log belongs to which part of the package restore.