Add OptionsHandler to TreeMux & node.setHandler

[derekperkins]

No description provided.

[derekperkins]

I squashed my commits from #18 and it closed the PR, not quite sure what happened. Here it is again as 1 commit. :)

[dimfeld]

I think this will make it impossible to set a different OPTIONS handler on any node, and it will also cause a difference in behavior if t.OptionsHandler is set before or after a route is added. Instead of setting this in the tree nodes, how about we just handle this in the router code after it has found a node. I'm thinking something like this, at router.go:311

if !ok {
    if r.Method == "HEAD" && t.HeadCanUseGet {
        handler, ok = n.leafHandler["GET"]
    } else if r.Method == "OPTIONS" && t.OptionsHandler != nil {
        handler = t.OptionsHandler
        ok = true
    }

    if !ok {
        t.MethodNotAllowedHandler(w, r, n.leafHandler)
        return
   }
}

[derekperkins]

With my branch, you'd be able set a custom OPTIONS route first, but you're right, that'd be unexpected behavior. I like your solution much better.