Added support for ConfigurationSetting<Enum>

This is to make to below scenario possible.


**[web|app].config**
```xml
<appSettings>
  <app key="EnviromentTypeConfigurationSetting" value="Local" />
</appSettings>
```

**possible implementation**
```csharp
enum EnviromentType 
{
  Local,
  Development,
  Staging,
  Production
}

EnviromentTypeConfigurationSetting : ConfigurationSetting<EnviromentType> 
{ 
}
```