Add a wrapper for CompactDisk of a VHD

[Roemer]

I added a wrapper methods for CompactVirtualHardDisk.
As this process can take a bit longer, I created two wrappers:

• CompactDiskAndWait which calls compact and waits until it is finished
• CompactDisk which returns the created job object where the caller is responsible for closing the job object and waiting until it is finished. This has the benefit that the caller can for example print out a progress with something like:

job, err := mgmt.CompactDisk(diskPath, constant.VirtualHardDiskCompactModeFull)
if err != nil {
	return err
}
defer job.Close()
for !job.IsComplete() {
	fmt.Println(job.PercentComplete())
	time.Sleep(100 * time.Millisecond)
}

I also added a constant for the various compact modes.

[madhanrm]

can you add some unit test in _test.go