Add Follow/Unfollow

* Add Follow and Unfollow
* User visit profile page which...
 * myself, show "Edit" link.
 * other's who has not be followed, show "Follow" button.
 * other's who has already be followed, show "Unfollow" button.
* Follow/Unfollow is ajax request. When ajax completed, toggle Follow/Unfollow button.

@adorechic Please review it!