Docs and API Reference generation with docfx

[markvincze]

This PR adds documentation and API Reference generation using docfx. The way it works is that a new docs folder contains all the config and setup. The docs can be generated with the docfx CLI, which puts the output (by default) to docs/_site. The idea is to host the docs with GitHub pages in the same repo in the gh-pages branch. The added releaseDocs.sh script self-contained, it generates the docs, checks out the repo, adds the new docs and pushes them to the gh-pages branch. For now I've only been executing this script in my machine. If we want, in the future we can set this up so this runs on AppVeyor automatically. Ignore everything in the docs/_exported_templates folder, that's just the default template docfx is using. I added it to the repo, because I wanted to customize the navigation bar a bit. What I did with the docs is that I copied everything from the README to the docs, and split into a couple of pages. I also adjusted the introduction docs a bit, you can find it in docs/documentation/intro.md (or on the site at https://travix-international.github.io/Hystrix.Dotnet/documentation/intro.html). I didn't change the README yet, I think in a followup PR we can make it shorter, and add a link saying "details in the documentation". I pushed the docs already to the gh-pages branch, so you can take a look at it here: https://travix-international.github.io/Hystrix.Dotnet/index.html

Addresses #21.

[1kastner]

That looks nice!

[markvincze]

I made some small changes, and I made the README shorter and pointing to the Documentation.
@JorritSalverda can you take a last look here if the new shorter README looks good? https://github.com/Travix-International/Hystrix.Dotnet/tree/docfx-generation