Add local demo

[thenewguy]

The installation docs are correct, but make it seem much more complicated to install the validator than it is.

I've added vagrant to the repo. This makes the demo usable with one command from the user.

I.e. run vagrant up from the vagrant subfolder and then you have a working demo for validating your DASH manifests.

[rbouqueau]

Thank you for this effort, I'm sure it's been useful to many.

@Phencys I think this has been superseeded, can you confirm and close?