Include initialization time in the total session time of the build log

This patch refactors the frontend App object and touches a few internals.

  o The scheduler now takes a start time given to it at instantiation time,
    instead of considering the start time to commence in Scheduler.run()

  o The App.initialize() method has been swapped out for an
    App.initialized() context manager.

    This context manager now takes care of the main start/fail/success
    messages. For convenience and ensured consistency, this context manager
    is now responsible for:

      o Printing the startup heading
      o Printing the end of session summary
      o Exiting with a consistent error status in case of errors

  o The Pipeline() object no longer prints the start/fail/success messages,
    as they are now taken care of by the App()

  o The cli.py frontend code is much simplified by using the new context
    manager, also enforcing consistency in how we handle and report errors.

This fixes issue #186