Code Related to Application Domains

The implementation of cross-AppDomain support for all classes proved very cumbersome and promised little benefits for the current goals of the library.

Therefore, I have decided to skip the cross AppDomain feature for the time being. The code will remain in the repository, of course, and if anyone is interested, the revision with the latest state of the AppDomain-related code is tagged as 0.1.8x-AppDomainProxies.