Update package to match new structure for accounts packages

I've updated the package, completely based on the most recent accounts-github package from Meteor.

Every accounts package is now (since Meteor 0.6.4) split into to packages, so I've created a new package to handle the Oauth flow (again, completely based on the current github package in Meteor).

This package is already [released on Atmosphere](https://atmosphere.meteor.com/package/foursquare), so if this pull request is accepted and published to Atmosphere, the two packages should work together to enable Foursuare authentication.

Thanks.