Support for a optional PartitionKey prefix

To allow multiple applications to share the same Table storage it could be usefull to have a prefix on the partitionkeys separating the source applications.

Added support for an optional configuration value both as a hard coded value and as a key reference to either the cloud config file or to the app/web.config appSettings section.

Sorry tool late but:
CloudConfigurationManager.GetSetting already fall back to AppSettings