Update angular-spinner.js

Hey
Great start with this project. I added several features that I needed for my project.

- Add support for configuration to allow something likw `<span us-spinner="small"></span>`
- Add an option to make the `<span us-spinner></span>` element large enough to fit the spinner (to make sure it won't be on top of other elements)
- Use an isolated scope for spinner directive.
- Save spinner in a local variable instead on the scope.