LibCore: Put all classes in the Core namespace and remove the leading C

I've been wanting to do this for a long time. It's time we start being
consistent about how this stuff works.

The new convention is:

- "LibFoo" is a userspace library that provides the "Foo" namespace.

That's it :^) This was pretty tedious to convert and I didn't even
start on LibGUI yet. But it's coming up next.

Applications/ChanViewer/BoardListModel.cpp

@@ -44,7 +44,7 @@ BoardListModel::~BoardListModel()
 
 void BoardListModel::update()
 {
-    CHttpRequest request;
+    Core::HttpRequest request;
     request.set_url("http://a.4cdn.org/boards.json");
 
     if (m_pending_job)