Simplify HTTP API and refactorings

Main changes are:

 * Changed `HttpRequestHeader` and `HttpResponseHeader` to modules
 * Added documentation comments for most HTTP headers
 * Renamed `dontThrowOnHttpErrors` to `silentHttpErrors`

I moved things back to `FSharp.Data`. I do not mind that there is a few `HttpFooBar` things (I just wanted to avoid the case where opening `FSharp.Data` would import fairly generic header names etc.). I moved things in modules which means that you can still open `FSharp.Data.HttpRequestHeaders` to see the names of headers directly - but if you mix requests & responses, you'll have to disambiguate.