Merge of steamgo into go-steam

These are the main sections of my code that should be merged into your project.
`enums.go` and `messages.go` are autogenerated, but `steamlang.go` is not. The calls would have to be changed in the generator though, that's right.

You'd just need to add an import [here](https://github.com/Philipp15b/go-steam/blob/75ea9f0a2cb23072af0a4ff66a28178058509185/generator/GoSteamLanguageGenerator/GoGen.cs#L61) and change all calls of those methods in that file too.

I can do that along with the regeneration later.