Move vagrant user's username into variables.

This makes it easier to use the prepare_iso.sh script to set up an ISO with
a different username and password, allowing it to be used to provision
non-vagrant images.
Remove explicit mention of vagrant in comments.
Include useful script to generate a shadowhash file.
Add optional command line switches to prepare_iso.sh.

prepare_iso.sh now accepts three optional command line switches to modify the
user, password and image of the root user in the prepared image.
Point to the correct vagrant.jpg path.
Fix username template in sudoers file.
Include vagrant.jpg.
Extract vagrant settings into user variables.

The defaults still build a vagrant compatible image as before, but now there
are variables to configure the username, password, whether the user is
automatically logged in, and whether the vagrant SSH keys are installed.
Merge branch 'master' into easier-renaming-of-vagrant-user
Use user variables in parallels builder.
Initial support for building Yosemite guests

- In addition to growing BaseSystem and copying over Packages, we
  also copy the BaseSystem dmg and chunklist files to the root
  of this new grown BaseSystem (yes, a copy of its original at the
  root of itself).
  - My guess would be that this is so that the installer can use
    these to do recovery partition-related tasks, verification, etc.
    because without it, the installer would fail complaining that
    it could not verify the recovery partition.
  - Cribbed from the 'convert_mavericks_image' script from
    Parallels 10 - a much simpler approach than VMware Fusion's
    approach of setting up a second disk with a custom boot config.
- Fixes #17.
pkg postinstall ssh enable: new launchd overrides path on 10.10
Update README.md
Rename user variables.
Set parallels tools path dynamically.
Remove unused kcpassword file.
Document set_kcpassword.py.
Merge branch 'master' into easier-renaming-of-vagrant-user

Conflicts:
	prepare_iso/support/pkg-postinstall
Default autologin to false.
Document prepare_iso.sh switches.