adds really simple Martini + Facebook example

Been playing with getting this working using [martini](https://github.com/go-martini/martini). I had to use regular GET params to set the provider (e.g. `?provider=facebook`) so that the builtin `GetProviderName` works. It would be nice to use [`martini.Params`](http://godoc.org/github.com/go-martini/martini#Params) instead but I haven't quite figured out a nice way to override `GetProviderName` and pass that in (still new to Go).