Fix a bug of extension type's const str & refactoring
Re-check in the xsd/cmd files to address CR
Address CR
Remove the IEnumerable template interface from the OutputType of Get-AzureServiceExtensionImage cmdlet

Also did it for the Get-AzureVMImage/Get-AzureDisk cmdlets to fix issue #1213.
Help texts for Get-AzureServiceExtensionImage, Get-AzureServiceADDomainExtension, & Remove-AzureServiceADDomainExtension
change file name for Set-AzureServiceADDomainExtension & remove default property value settings: 'Mandatory = false'
Merge pull request #69 from huangpf/addomain

ADDomain Extension Cmdlets