.net standard support

This version is functionally equivalent to v1.1.0, but targets both .Net Framework 4.5.1 and .Net Standard 2.0.
The tests have been converted to xUnit and runs on both .Net FrameWork and .Net Core.

The only change to the codebase is how the polling background threads are stopped when the listeners are closed. Previously this was done by aborting the thread, but now a proper CancellationToken triggers a graceful shutdown of the thread instead.

Initial release

v1.0.0

Added reference to Wiki-pages for API-documentation