Add this directive to a button or link in order to authenticate using a social provider.
The value should be the account store HREF a social provider, such as Google or Facebook.
The sp-name field must be set to the provider ID of the corresponding provider, e.g.
google or facebook.
Any additional fields can be specified as an object, via the sp-options field. These
options will additionally be augmented (and overriden) by the options set for the given
provider (determined by value of sp-name) in STORMPATH_CONFIG.SOCIAL_LOGIN_OPTIONS.
http://docs.stormpath.com/guides/social-integrations/