moving default basepath to var, convenience functions to change/creat…

[fredim]

…e index

hey - love the project. so, my own server has quite an extensive list of handlers, and I just want to put this under a deeper directory structure. actually, I just needed the NewIndex function, but i added the RootDir stuff in case it was useful.

[arl]

First of all thank you for your interest in the project and for that pull-request.
Also I think that's a great addition

There are some new statsviz features in the making which are going to require some additional configuration, and I think being able to change the root will fit nicely into the API I have in mind.

Once I'll have commited the new API, I'd be delighted to integrate your PR.
I'll tag you here once it's done so you can adapt the code to the new API.

How does that sound?

[arl]

@fredim
I have just merged the feature I was talking about.
You can now change the root by calling Register(mux, Root("/your/new/root").

[arl]

So i'm closing since the feature provided by this PR exists already.