get_historical_usage_metrics and get_metric_definitions

Add "get historical usage metrics" for a Web Site:
http://msdn.microsoft.com/en-us/library/azure/dn166964.aspx

The "time_grain" option is not documented in the official library, but I inferred it from this Azure .NET issue:
https://github.com/Azure/azure-sdk-for-net/pull/523
and from error message which tell me to change the "time grain" (how I do that without an option? :))

Add "metric definitions" request:
http://msdn.microsoft.com/en-us/library/dn448676.aspx