Add a retrieving binary data from memory (instead of files on disk) function on martini.Static.

Hi, I changed `static.go` so that `martini.Static` can serve binary data from memory instead of files on disk.

I wanted to serve binary data generated from static files by using [go-bindata](https://github.com/jteeuwen/go-bindata) and I added this feature on `martini.Static`.

Thanks.