Add progress aggregator for govc

This will be used to group progress of a sequence of tasks in one line.
Merge pull request #73 from pietern/progress-aggr

Add progress aggregator for govc
Include Client.URL's port in ParseURL
Stop Ticker in ProgressLogger
Ignore PowerOff error in vm.destroy command
Add progress for ovf datastore.import

Progress is used both for updating NfcLeaseProgress and
cli ProgressLogger