Sql Databases Management Service list commands

Create a SQL Database Service Management class

2 operations for now:
- list_servers : http://msdn.microsoft.com/en-us/library/azure/dn505702.aspx
- list_databases : http://msdn.microsoft.com/en-us/library/azure/dn505711.aspx

This commit adds a new parser for `ServiceResources` xml format used in SQL REST API with the method `_parse_service_resources_response`

Usage template:
```python
response = self._perform_get(self._get_list_databases_path(name), None)
return _parse_service_resources_response(response, Database)
```
This can be used  to implement all remaining SQL REST API command