DSC Extension: ConfigurationArgument support for PSCredential

Changes:

1) Add support for Set-AzureVMDscExtension -ConfigurationArgument { Credential = Get-Credential }
2) Update Protocol for AzureVMDscExtension (Properties was a Hashtable, now it's an array)
3) Add comments to public classes
4) Update help file to reflect PSCredential support

We are targeting 9/10 release for this changes.
We may send few bugfixes, tests and docs updates during next week. What is a deadline for dev -> master merge for this release?