Improved support for portable class libraries

These commits add the following features to your portable library implementation.

* LogManager.GetClassLogger() support

This works on Silverlight and .Net, but will throw PlatformNotSuported on Windows Phone where the functionality is unavailible.

It is possible to add windows phone support, but then we would have to dynamically create and use the StackTrace on each time which can have significant overhead.

* Works with .config files

ConfigurationSectionHandler and DefaultConfigurationReader are updated so that they are able to read configuration when running on .Net, for silverlight, and windows phone the approach is the same as previously with an empty configuration