Allow inserting OWIN middlewares before the WebSharper sitelet

[psfblair]

These commits allow middlewares to be inserted into the OWIN pipeline before the WebSharper sitelet; the test application now contains an example that uses OWIN logging middleware to log requests and responses.

These changes do not allow for Middlewares to be inserted in the pipeline after the WebSharper sitelet because SiteletMiddleware generally swallows the call to Next, and how to address that wasn't clear. However, the response logging example shows how the output from the sitelet middleware can still be handled by a middleware that is inserted prior to it in the pipeline.

Two additional notes:

1. Commit e615b7c introduces a MiddlewareGenerator type in order to allow the IAppBuilder to be passed to middlewares that need it (e.g., middlewares using the OWIN logging framework). This type should really be migrated to WebSharper.Owin once this PR is merged.
2. The WebSharper.Warp.Test test server now makes use of an OWIN logger. However, using it requires that the WebSharper.Warp.Test.exe.config file be present in the same directory as the executable. This works when compiling using Visual Studio into the bin\Release and bin\Debug directories, but I haven't been able to figure out how to get IntelliFactory.Build to copy the app config file into the build\net45 directory as part of the command-line build. Assistance on that would be appreciated.

[psfblair]

Apologies for the line ending issues.

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❌ Paul Blair
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