Add option to isolate store to AppDomain rather than to assembly.

This fixes the "System.IO.IOException: The process cannot access the file 'manifest.xml' because it is being used by another process." issue when the one ASP.NET application is mapped to more than one IIS application, and two worker processes try to rebuild the bundles at the same time.
Slight refactoring - Moved logic from constructor into method
Merge branch 'isolated-storage-appdomain' of git://github.com/Daniel15/cassette into Daniel15-isolated-storage-appdomain