Port over the rest of a tale of 3 nightclubs

[battermann]

14 Done

To-Do

• [x] port test 2 (partially)
• [x] add extension method Join to support C# LINQ query syntax
• [x] add source code comments
• [x] update API reference
• [x] update C# tutorial "A Tale of 3 Nightclubs"
• [x] port the rest of part 2
• [x] port part 3

[battermann]

Done.

I built some things back in the end because:

• The method ReturnValidation implies a default message type of string which serves well the C# tutorial but probably isn't very helpful in production because message types will most likely be custom composite types or enums.
• I was not sure if currying should be part of this library or not. I tend to think not.
• CostByGender takes more arguments of type Person than needed just because it has to match the number of checks. I think that is inconvenient and counterintuitive.

So what is left in this PR is really the core. Minimally invasive. That is the Join extension method and everything from the original tutorial that could be done without changing anything else.

[forki]

this is great! Thanks for helping with this.