Mention requirement of VirtualBox command line utilities

VboxManage is a requirement and may not be installed. Mention this in
the README and VirtualBox checking function error message.
Merge pull request #113 from nicklevett/commandlineutils-message

#112 Mention requirement of VirtualBox command line utilities
Added check for unloaded kernel module in version output

Fixes #107

When the kernel module is not loaded an error message is output when
running VBoxManage -v (though the exit code is still 0). This prevents
$major_minor_release from being assigned correctly. The script will exit
with the following error message:

bash: line 43: WARNING: unbound variable

The error message contains instructions on how to load/rebuild the
kernel module for the given platform, so we can just exit and output
that message verbatim. The following is an example of the message on
Fedora:

WARNING: The vboxdrv kernel module is not loaded. Either there is no module
         available for the current kernel (3.7.3-101.fc17.x86_64) or it failed to
         load. Please recompile the kernel module and install it by

           sudo /etc/init.d/vboxdrv setup

         You will not be able to start VMs until this problem is fixed.
4.1.24r82872
Merge pull request #115 from whiteinge/master

Added check for unloaded kernel module in version output
Fix IE6 auth loop with hotfix. closes #105
Update readme with IE6 hotfix info and video.
Convert readme to Markdown.
Use modern.IE VMs! :joy:
Use IE6 VM for IE7 and IE8 by default and use unar instead of unzip on OS X.
Add iso control files.
Fix IE10 disk unit.
Typo.
Fix unar variables.
Update readme for XP VM reuse.
Fix units for IE7 and IE8 when REUSE_XP is off.
Add acknowledgements to readme.
Bail on xp build if REUSE_XP is off.
Ignore stuff.
Acknowledge uck.
Merge branch 'develop'