Added a way to wrap an already created AppDomain in an AppDomainToolkit.AppDomainContext

In one of my projects which uses AppDomainToolkit I want a quick and easy way to turn *off* using AppDomains and just to load everything into the main domain.

This PR seems to be the easiest way to do this, it allows me to keep all my code the same except where I create the domain I just switch:

    _context = AppDomainContext.Create();

To:

    _context = _pluginContext = AppDomainContext.Wrap(AppDomain.Current);

Does this seem like a sensible thing to be doing?

Signed-off-by: Martin Evans <martindevans@gmail.com>