Put service data in user.services.instagram instead of user.profile

I noticed this inconsistency while using your package in my [Avatar](https://github.com/bengott/meteor-avatar) package. This PR modifies your code to follow the pattern of the other (core) accounts login service packages (i.e. set user.profile.name using data from the service, but put all other service data in user.services.serviceName).

I also added a readme file. Nothing much in there yet. ;)