Dev

New cmdlets:

Get-AzureVMAvailableExtension
Get-AzureVMBGInfoExtension
Set-AzureVMBGInfoExtension
Remove-AzureVMBGInfoExtension

Modified cmdlet:

Add-AzureProvisioningConfig now enables GA in default and has -DisableGuestAgent parameter for disabling GA.

...And some bug fixes.