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
Merge pull request #72 from dougm/nfc-lease-updater

Nfc lease updater
Improve about command output
Add datastore.import support for .ova files
Merge pull request #74 from dougm/ova

Add datastore.import support for .ova files
Add DeleteVirtualDisk function
Be specific about channel direction
Support importing VMDKs into ESXi machines
Merge pull request #75 from pietern/import

Support importing VMDKs into ESXi machines
Add NewResourcePool func
Change NewDatastore signature

Not currently used.  Use same sig as the other govmomi types.
Add NewFolder func
go vet: composite literal uses unkeyed fields
go vet: format related warnings
Rename OutputWrite.WriteTo method to Write

Avoids:
 go vet: error should have signature WriteTo(io.Writer) (int64, error)