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.

This method should be just:
```
public string GetSettingByKey(string key)
        {
            //try get string from app settings or cloud service config
           return CloudConfigurationManager.GetSetting(key);
        }
```