Custom build task for copying documentation comments added

I'll get up a repo for this build task some day soon to make the source available, it's still a little ugly as the whole thing is built by trying to create a DSL around Roslyn's types using extension methods. So yeah, the shame runs deep.

But for now it would be great if someone just wanted to pull down this branch and give it a go, see if they experience any issues with it, I think I ironed most of them out as I'm finding it difficult to think of any more edge cases to test.

Also didn't want to commit the files that it changes quite yet since some of the replaced comments imo were better than the ones on the interfaces, I'll leave that decision to someone else unless it's put in my hands.

Once I have a repository up for this I might also add it to nuget so that the binaries don't have to be included  here. Although I think that might be somewhat further down the line as I imagine I ought to make it a lot more flexible before that.