Add support for polling performance counters of VirtualMachine objects

* New method for polling performance counters: vm.counter.get
* New method for getting all performance counters: vm.counter.all
Add methods for polling performance counters for HostSystem objects

* New methods added: host.counter.get & host.counter.all
Document the new vm.counter.* and host.counter.* methods
vpoller-client accepts polling with or without providing any properties

* This is required during polling performance counters as
  getting all performance counters for an object does not require you to
  provide any counter names
Add -D|--debug command-line option to the Proxy/Client/Worker
Be more verbose when in DEBUG mode