Feature/user stats

* Adds a rake task to export a csv listing the users, with names (MD5'd), creation date, and number of groups belonged to. 
* Adds number of groups belonged to to admin users panel
* Makes the stats tasks export csvs to an S3 bucket rather than to the tmp folder (tested on http://loomio-mhjb.herokuapp.com). This needs a aws.yml added to config with access keys. It looks like we could share the bucket (associated with my own s3 account) with loomio's s3 account (http://stackoverflow.com/questions/6412702/is-it-possible-to-share-a-amazon-s3-bucket-between-amazon-s3-users). Or might make more sense just to change stats.rake to use a bucket that already belongs to loomio's s3.

Still a total junior so all of the above likely to have been done wrong.