FEATURE: export user list

This PR adds feature to export a user list in a CSV file from `/admin/users` page. Summary of the feature:

The Admin clicks on *Export Users* button:

![export_user_1](https://cloud.githubusercontent.com/assets/5732281/3923850/14a6260c-23d7-11e4-8c07-06cce3d1fd7d.png)

He will see an alert:

![export_users_1 1](https://cloud.githubusercontent.com/assets/5732281/3923863/4b21b372-23d7-11e4-8518-95442e3ce5cb.png)

Shortly he will receive a PM:

![export_user_2](https://cloud.githubusercontent.com/assets/5732281/3923860/41e7529e-23d7-11e4-930f-e91046bdeaed.png)

Clicking on the attachment link, CSV file will be downloaded.

Currently only email field is exported.

![export_user_3](https://cloud.githubusercontent.com/assets/5732281/3923989/7fd5fc6c-23d8-11e4-92ac-039a34cc5c60.png)

Please provide feedback.
I was going to suggest using params.permit for this API, but it looks like you don't ever call this method with params anyway. Should it just be `AdminUserIndexQuery.new` instead?