Fix/call opt shutdown script from init

Fixes #289.

#### Changes

##### Update rc.shutdown from make_iso.sh
`make_iso.sh` already extracts TCL's rootfs, so this is where I modify `/etc/init.d/rc.shutdown`.  The sed script seems intimidating, but for good reason.  I can explain how the sed command works if anyone is interested.

I choose to insert the "test && run" command after `rc.shutdown` completes initialization in case future development decides to display console messages from `shutdown.sh`.  Sed will:

1.  Insert "test && run" after the `useBusybox` and `clear` lines.
2.  Only insert once.

The sed range `1,/^useBusybox/` protects against multiple lines of `useBusybox`&mdash;followed by a blank line then followed by `clear`&mdash;in `rc.shutdown`, but this may be overkill, because there's no reason for someone to do that.  I think the range is a small price to pay, but I can elide this if requested.

```diff
--- rc.shutdown.old
+++ rc.shutdown
@@ -5,6 +5,10 @@
 useBusybox

 clear
+
+# Run boot2docker shutdown script
+test -x "/opt/shutdown.sh" && /opt/shutdown.sh
+
 # Sync all filesystems.
 echo "${BLUE}Syncing all filesystems."
 sync; sync
```

##### Fail build if rc.shutdown doesn't change
The previous change is fragile in that, any changes to `rc.shutdown` may break the pattern I use to insert the "test && run" command.  If a grep check fails, then the build process stops and prints an error message.  This ensures that future changes don't disable the "test && run" command without notice.

##### Update the ACPI power action
We no longer need `exitcheck.sh` here.  It does two things: 1) run `/opt/shutdown.sh` and 2) run `/sbin/poweroff`.  Leaving it will result in the shutdown script running twice.  Calling `/sbin/poweroff` instead.

##### Print a message during shutdown/reboot
The future is here!  `/opt/shutdown.sh` will print "Running boot2docker shutdown script..." to the console.  I found this useful for testing this changeset.

#### Testing
* Over SSH:  Ran `poweroff` and `shutdown` from within a boot2docker VM on VMWare ESXI 5.1.  Passed.
* ACPI (I think):  Loaded boot2docker in KVM/QEMU, opened qemu-monitor, and ran `system_poweroff`.  Passed.
* Build stops with error if `rc.shutdown` does not contain `/opt/shutdown.sh`.  Passed.
* Build continues if `/opt/shutdown.sh` was inserted into `rc.shutdown`.  Passed.