Added configurable default options

Hey,

I added some configurable options with a provider that creates usSpinnerConfig object. You can now configure defaults for any spinner on application startup, they can still be overridden with the directive options. Karma tests are included in the PR. 

Example of configuration:

    angular.module('myApp', ['angularSpinner'])
      .config(['usSpinnerConfigProvider', function (usSpinnerConfigProvider) {
        usSpinnerConfigProvider.setDefaults({color: 'blue'});
      }]);