Move StatusCodes to a more visible namespace

[rynowak]

Right now it's in Microsoft.AspNet.WebUtilities namespace. Suggest moving it to Microsoft.AspNet.Http so that it's more easily discoverable.

Example: https://github.com/aspnet/Mvc/pull/1924/files

We're using this in plenty of places in MVC but it's pretty much the only from that namespace we're interested in.

[Tratcher]

I wouldn't want to change the namespace without also changing the package. Maybe we could put it in the Microsoft.AspNet.Http.Abstractions package and the Http namespace?

I noticed that very few of the linked samples referenced the Microsoft.AspNet.Http namespace either, so moving there doesn't really save you anything.

I'd like to put it into Microsoft.Net.Http, but I don't think they'd go for that.

[Tratcher]

Or should we rename this package to Http.Utilities?

[Tratcher]

Put it in Http.Abstractions package and Http namespace.

[Tratcher]

@rynowak Can you handle your own cleanup for this?