Update to newer tooling

- move to MSBuild v15.0
- move to StyleCop.MSBuild v5.0.0
  - stable release that supports C#6, not just `nameof(...)`
- move test output path beside, rather than under, regular output path
  - workaround NuGet/Home#6001
  - `\$(Configuration)\Test` -> `\Test\$(Configuration)`
  - quiets builds down significantly
- add AppVeyor configuration and status badge
- disable Travis builds for now
- ignore launchSettings.json files

ExecutionDataflowBlockOptions were not passed into all ActionBlock in…

…stances in DataFlowWebHookSender making some of them run with a default MaxDegreeOfParallelism of 1.

Incremented timeouts for some tests that could time out on build machine

Added sample showing how to use ASP.NET WebHooks with a dependency in…

…jection engine. In this example we use autofac but others are equally valid.

Changed to consistent use of nameof(...)

Updated doc comment.

Made it possible to hook into the default WebHookSender instance and …

…get called when a WebHook retries (in addition to succeeds, is gone, or fails). This enables for a variety of post-send processing scenarios where an implementation may want to take certain actions depending on whether a WebHook is retried.

Update README.md

Added announcement for RC1

Enforce mandatory properties in Azure Alert payload

- Tag mandatory properties with Required.Always so JSON.NET enforces the requirement during (de)serialization
- Fix a couple of comment typos on KuduNotification
- Add required 'resourceRegion' property to AlertMessage1.json test content
- Add AlertMessage3.json to validate change against sample payload in Azure docs
- Add unit tests

Minor code tweak