Silverlight port

Plans for a Silverlight version have been dropped due to lack of demand.

If someone is looking to restart this work, SL-specific build and conditional compilation was removed in these commits:

• https://github.com/nsubstitute/NSubstitute/commit/9b8561ec75f5146baedb98925ec181c8147e1cab
• https://github.com/nsubstitute/NSubstitute/commit/1ec10818772d6597e69fb427cc4d2365b200b9ae

See wiki page history for previous status.