Add help for Set-AzureVNetGatewayKey
Merge branch 'dev' of https://github.com/WindowsAzure/azure-sdk-tools into juneb-140502-01

Conflicts:
	WindowsAzurePowershell/src/Commands.ServiceManagement/Microsoft.WindowsAzure.Commands.ServiceManagement.dll-Help.xml
Merge pull request #2515 from stankovski/juneb-140502-01

Add help for Set-AzureVNetGatewayKey