Fix issue #249 with AppSettings provider

Hi

please review this pull request. It contains changes needed to fix issue #249. 
Now AppSettings type provider returns fresh settings from <app_name>.exe.config each time app or dll loaded.

Before change settings were stored as static constants and now each setting key is a static property with same name as static constant. This is needed to read values from app.config not in compile-time, but in run-time - getter calls ConfigurationManager.AppSettings["some key ..."]. 

However on the source-level changes are backward compatible: "FSharpx.TypeProviders.AppSettings.Tests" project could be re-compiled without any code change.