Avatars in notifications are now circular.

Added wrapper view around noticon in notifications list to simulate a border. (setting cornerRadius and border on the layer were causing some artifacting)
Restoring `noticonRadius` property.
Added circular gravatars to the Reader and comments
Increased the size of the notion by a few pixels.

Design states that noticon should be 16px.
Merge branch 'develop' into issue/2654-circular-avatars
Added `CircularImageView` subclass to fix some DRY.
Merge branch 'develop' into issue/2654-circular-avatars
Set the frame on the imageView in `CommentContentView`.

Allows the circular rounding to work properly.
Merge branch 'develop' into issue/2654-circular-avatars
Corrected WPContentAttribution view to use CircularImageView.

Code style cleanup, hat tip @aerych.
CircularImageView now used in ReaderPostHeaderView
Merge pull request #2744 from wordpress-mobile/issue/2654-circular-avatars

Circular Avatars