xMove-VM.ps1 usability and PowerShell convention updates

[dbaileyut]

• Update comment based help with shorter synopsis and examples
• Add help comments for parameters
• Get rid of need to define variables outside the scope of the function
• Use PSCredential to help discourage saving credentials in the clear
• Remove unused $switch parameter
• Add Folder parameter
• Make booleans switches
• Connect to vCenters within the function (only if not already
  connected)
• Use the SSL thumbprint in the vCenter connection to eliminate the need
  of mucking with certificate validation in .NET
• Improve parameter validation and return from the script if unexpected
  results are found
• Allow for multiple destination datastores
• Add Async switch to avoid waiting on the task