add current sign-in date

Add current sign-in date to admin section profile view.

![image](https://cloud.githubusercontent.com/assets/1192780/4026899/75867f76-2c24-11e4-9b34-ffc60b50ce47.png)

I'm a bit confused why both `current_sign_in_at` and `last_sign_in_at` exist but regardless they do not match and as such I believe both should be shown.