Automatic selection of most recent gallery templates

This change allows the selection of the most recent gallery templates based on version when a publisher or template name or partial name is supplied.
Still don't understand the reason of having this parameter. If user executed:
```csharp
Get-AzureResourceGroupGalleryItem
```
This means every All/Everything up there.

In your case, user can do this to get all latest gallery items:
```csharp
Get-AzureResourceGroupGalleryItem -Latest
```